fix: Fix error messages when a comprehension iterates over a non-array.

[rudo-thomas]

Problems:

• When iterating over an empty string in a list comprehension, the result is an empty string. This is a bug, it should be an error.
• When iterating over a non-empty string in a list comprehension, the expected and unexpected types in the error message are swapped.
• Error messages mention "std.flatMap" when object/list comprehensions would iterate over a value that is neither array nor string.

$ jsonnet --version
Jsonnet commandline interpreter (Go implementation) v0.21.0-rc2
$ jsonnet -e '[a for a in ""]'
""
$ jsonnet -e '[a for a in "b"]'
RUNTIME ERROR: Unexpected type array, expected string
	<cmdline>:1:1-17
	During evaluation
$ jsonnet -e '{[a]: 1 for a in 2}'
RUNTIME ERROR: std.flatMap second param must be array / string, got number
	<cmdline>:1:1-20
	<cmdline>:1:1-20
	During evaluation
$ jsonnet -e '[a for a in 1]'
RUNTIME ERROR: std.flatMap second param must be array / string, got number
	<cmdline>:1:1-15
	During evaluation

FWIW, the C++ implementation does not have any of these problems. It gives:

RUNTIME ERROR: In comprehension, can only iterate over array.

In the Go implementation comprehensions are desugared to a call to std.flatMap which does accept a string in the "arr" parameter.

The fix: Desugar comprehensions to a call to a new hidden builtin which only accepts arrays.

[rudo-thomas]

@johnbartholomew
Maybe this could still make it into 0.21.0-rc3 (or final)? PTaL 🙏

[coveralls]

coverage: 55.582% (+0.008%) from 55.574%
when pulling 1c79f57 on rudo-thomas:rudo-fix-comprehension-over-string-error-message
into 1add1e1 on google:master.

[johnbartholomew]

Thanks for finding & fixing this. The implementation looks good to me.

[johnbartholomew]

Rebased & merged