Regression in stack usage from 0.17 to 0.18

There seems to be a regression in the use of the stack from version 0.17 to version 0.18. I have been able to reproduce with a simple bit of jsonnet code. Notice how it works fine in version 0.17 with a max stack of 128, but causes max stack frames to be exceeded in version 0.18 unless I up the max stack to 2005

```
[C:\>]$ gojsonnet17.exe -v
Jsonnet commandline interpreter v0.17.0
12:19:43 [ppodaril@ppod-base C:\g\A\F\idsr]$ gojsonnet17.exe -e --max-stack 128 '{ \"a\": std.foldl(function(p, c) p{ \"test\"+: { \"t\": c }}, std.range(0, 2000), {}) }'
{
"a": {
"test": {
"t": 2000
}
}
}
[C:\>]$ gojsonnet18.exe -v
Jsonnet commandline interpreter (Go implementation) v0.18.0
[C:\>]$ gojsonnet18.exe -e --max-stack 128 '{ \"a\": std.foldl(function(p, c) p{ \"test\"+: { \"t\": c }}, std.range(0, 2000), {}) }'
RUNTIME ERROR: max stack frames exceeded.
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
...
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
:1:45-55 +:
Field "test"
Field "a"
During manifestation

[C:\>]$ gojsonnet18.exe -e --max-stack 2005 '{ \"a\": std.foldl(function(p, c) p{ \"test\"+: { \"t\": c }}, std.range(0, 2000), {}) }'
{
"a": {
"test": {
"t": 2000
}
}
}
```