Unquoted integer keys in Map/Object will experience leading zero truncation

Based on documentation [here](https://www.terraform.io/docs/language/expressions/types.html#more-about-complex-types) and [here](https://www.terraform.io/docs/language/expressions/type-constraints.html) the keys for a map/object must be strings and if the key begins with a number it must be quoted. However the existence of unquoted numeric keys does not generate an error or warning and the numeric keys are currently accepted.

A complication arises when unquoted integers are provided as keys with the intent that they should be treated as string literals (as the documentation for maps/objects indicates that the keys are strings). If the integer has leading zeroes then they will be truncated and the key in the map/object will represent the truncated value even though in the terraform config it shows with a leading zero.

Technically this could be considered user error as the documentation does state that quotes must be used in this case when the key starts with a digit. Due to the confusion this could cause I would recommend also generating a warning or error when using unquoted numbers as keys.

### Terraform Version
```
Terraform v0.15.2
on linux_amd64
```

### Terraform Configuration Files
```terraform
locals {
foo = {
123 = "abc"
012 = "def"
}
}

output "a" {
value = lookup(local.foo, "012")
}
```

### Expected Behavior
```
Plan: 0 to add, 0 to change, 0 to destroy.

Changes to Outputs:
+ a = "def"
```

### Actual Behavior
```
╷
│ Error: Invalid function argument
│
│ on main.tf line 10, in output "a":
│ 10: value = lookup(local.foo, "012")
│ ├────────────────
│ │ local.foo is object with 2 attributes
│
│ Invalid value for "inputMap" parameter: the given object has no attribute "012".
╵
```

### Steps to Reproduce
1. Create main.tf with the above contents
2. Run terraform plan with either the 123 or 012 keys in the lookup

Start with the linked complex-types and type-constraints documentation, then reproduce the behavior using the Terraform configuration and lookup steps in the report. Confirm whether the intended outcome is a warning or error for unquoted numeric keys, or corrected lookup behavior, and document the chosen behavior when the issue is resolved.

Written by the indexing model from the issue text.