When struct keys are not strings, should error, not panic

When a non-string value has been provided for an key in a struct, `ytt` should report an error

```
ytt: Error:
- expected string for data value key, was bool
config.yaml:7 | on:
```

rather than panic (see below).

👇🏻 original report 👇🏻
---

when using `on` as a key in yaml (see below example) ytt panics. changing it to anything else solves it (just change the on: to on1: for example)

config.yaml:
```
#@data/values
---
general:
notifications:
recipients:
- admin@test.com
on:
- any
```
data.yaml:
```
#@ load("@ytt:data", "data")
- name: Delete
type: multistep
steps:
- type: ansible
inventory: hosts
#@ if/end hasattr(data.values.general,"notifications"):
notifications: #@ data.values.general.notifications
```

ytt panics with the following stack trace:

```
panic: expected struct key %!s(bool=true) to be string

goroutine 1 [running]:
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.dictAsStarlarkValue.func1({0x8924a0, 0xd61ba8}, {0x88fee0, 0xc00009b818})
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:100 +0x127
github.com/vmware-tanzu/carvel-ytt/pkg/orderedmap.(*Map).Iterate(0xaaaaaaaaaaaaa, 0xc00018cdf8)
github.com/vmware-tanzu/carvel-ytt/pkg/orderedmap/map.go:71 +0x79
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.dictAsStarlarkValue({{0x9004e0, 0xc00009b7b8}, {0xe8, 0x0}}, 0xda58e0)
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:96 +0xe9
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.asStarlarkValue({{0x9004e0, 0xc00009b7b8}, {0xf4, 0x0}}, {0x9004e0, 0xc00009b7e8})
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:83 +0x2fc
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.dictAsStarlarkValue.func1({0x896f20, 0xc00017ab90}, {0x9004e0, 0xc00009b7e8})
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:98 +0x98
github.com/vmware-tanzu/carvel-ytt/pkg/orderedmap.(*Map).Iterate(0x0, 0xc00018d008)
github.com/vmware-tanzu/carvel-ytt/pkg/orderedmap/map.go:71 +0x79
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.dictAsStarlarkValue({{0x9004e0, 0xc00009b7b8}, {0xd0, 0x0}}, 0xc00018d090)
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:96 +0xe9
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.asStarlarkValue({{0x9004e0, 0xc00009b7b8}, {0xf4, 0x0}}, {0x9004e0, 0xc00009b7d0})
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:83 +0x2fc
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.dictAsStarlarkValue.func1({0x896f20, 0xc00017ab80}, {0x9004e0, 0xc00009b7d0})
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:98 +0x98
github.com/vmware-tanzu/carvel-ytt/pkg/orderedmap.(*Map).Iterate(0x18, 0xc00018d218)
github.com/vmware-tanzu/carvel-ytt/pkg/orderedmap/map.go:71 +0x79
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.dictAsStarlarkValue({{0x9004e0, 0xc00009b7b8}, {0xb8, 0x0}}, 0xc00018d2f8)
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:96 +0xe9
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.asStarlarkValue({{0x9004e0, 0xc00009b7b8}, {0x8, 0x0}}, {0x9004e0, 0xc00009b7b8})
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:83 +0x2fc
github.com/vmware-tanzu/carvel-ytt/pkg/template/core.GoValue.AsStarlarkValue(...)
github.com/vmware-tanzu/carvel-ytt/pkg/template/core/go_value.go:36
github.com/vmware-tanzu/carvel-ytt/pkg/yttlibrary.NewDataModule(0xa84f00, {0xa84f00, 0xc00019daa0})
github.com/vmware-tanzu/carvel-ytt/pkg/yttlibrary/data.go:27 +0x45
github.com/vmware-tanzu/carvel-ytt/pkg/workspace.(*TemplateLoader).EvalYAML(0xc0001ae7e0, {0xc0000ba690, 0xc0000ba690}, 0xc0000ba5f0)
github.com/vmware-tanzu/carvel-ytt/pkg/workspace/template_loader.go:198 +0x47c
github.com/vmware-tanzu/carvel-ytt/pkg/workspace.(*LibraryExecution).eval(0xc00018da58, 0xc0001a82c0, {0x0, 0xc0001969f0, 0xc00018d7a0}, {0x0, 0xa96c88, 0xc00019ff40})
github.com/vmware-tanzu/carvel-ytt/pkg/workspace/library_execution.go:225 +0x91e
github.com/vmware-tanzu/carvel-ytt/pkg/workspace.(*LibraryExecution).Eval(0xc00018da58, 0x0, {0x0, 0x0, 0xc00009a948}, {0x0, 0xc00008b860, 0x0})
github.com/vmware-tanzu/carvel-ytt/pkg/workspace/library_execution.go:177 +0x46
github.com/vmware-tanzu/carvel-ytt/pkg/cmd/template.(*Options).RunWithFiles(0xc0000c7540, {{0xc00008b8b0, 0x2, 0x14}}, {0xa8a930, 0xc000135290})
github.com/vmware-tanzu/carvel-ytt/pkg/cmd/template/cmd.go:164 +0x570
github.com/vmware-tanzu/carvel-ytt/pkg/cmd/template.(*Options).Run(0xc0000c7540)
github.com/vmware-tanzu/carvel-ytt/pkg/cmd/template/cmd.go:100 +0x4b7
github.com/vmware-tanzu/carvel-ytt/pkg/cmd.NewCmd.func1(0x0, {0xc0000b89e0, 0x0, 0x0})
github.com/vmware-tanzu/carvel-ytt/pkg/cmd/template.go:22 +0x1d
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0x0, {0xc0000b89e0, 0x0, 0x2})
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0x6d
github.com/cppforlife/cobrautil.WrapRunEForCmd.func1.1(0xc000120a00, {0xc0000b89e0, 0x0, 0x2})
github.com/cppforlife/cobrautil@v0.0.0-20200514214827-bb86e6965d72/misc.go:45 +0x6d
github.com/spf13/cobra.(*Command).execute(0xc000120a00, {0xc00008c160, 0x2, 0x2})
github.com/spf13/cobra@v1.5.0/command.go:872 +0x624
github.com/spf13/cobra.(*Command).ExecuteC(0xc000120a00)
github.com/spf13/cobra@v1.5.0/command.go:990 +0x3bc
github.com/spf13/cobra.(*Command).Execute(...)
github.com/spf13/cobra@v1.5.0/command.go:918
main.main()
github.com/vmware-tanzu/carvel-ytt/cmd/ytt/ytt.go:21 +0xd6
```

**Environment:**
ytt version 0.43.0
CentOS Linux release 7.8.2003 (Core)

Start in pkg/template/core/go_value.go around dictAsStarlarkValue, where the stack trace shows the panic, and inspect the related iteration in pkg/orderedmap/map.go. Reproduce the issue with the config.yaml and data.yaml examples, then verify that the non-string struct key produces the reported error instead of a panic.

Written by the indexing model from the issue text.