Check that keys of assoc array literals are quoted

[andychu]

From #648

The following is the case where the type of the variable cannot be determined statically (which seems not be prohibited by Oil):

$ bin/osh -c 'v=A; declare -$v d=([a]=123); echo ${d["a"]}'
123

[andychu]

@akinomyoga I wrote a comment about this issue here:

https://github.com/oilshell/oil/blob/master/osh/word_.py#L518

Basically A=([k]=v) in Oil has to be an associative array; it can't be an indexed array.

However I can see it's a bit confusing nonetheless. We could add a check for it, but I noticed that there are tons of scripts that use it unquoted.

I guess we would hide it behind shopt -s parse_strict_array or something.