jsonnetfmt produces invalid output for object comprehensions with literal computed keys

[Lougarou]

jsonnetfmt formats a valid object comprehension into one that no longer parses. It strips the
[...] brackets off a string/identifier-literal computed key, but object comprehensions require
the [expr] key syntax, so the formatted output is rejected by the evaluator.

Repro (v0.22.0):

comp.jsonnet:

{['0']: 1 for x in [1]}

$ jsonnet comp.jsonnet
{
   "0": 1
}

$ jsonnetfmt comp.jsonnet
{ '0': 1 for x in [1] }

$ jsonnetfmt comp.jsonnet | jsonnet -
<stdin>:1:10-13 Object comprehensions can only have [e] fields

So formatting a valid file yields a file that fails to evaluate.

The bug is specific to comprehension keys that are simple literals. Complex keys keep their
brackets and stay valid:

$ echo '{[std.toString(0)]: 1 for x in [1]}' | jsonnetfmt -
{ [std.toString(0)]: 1 for x in [1] }      # correct, still parses

Root cause looks like the ['literal'] -> literal field simplification (correct for normal
objects, since {['a']: 1} == {a: 1}) is being applied inside object comprehensions, where
that rewrite is not valid. The fix would be to skip the simplification when the field is part of
a comprehension.

_{Found via a formatter idempotence check and confirmed with the repro above on v0.22.0.}