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

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.