Fixes #48.

I failed to find a comprehensive spec for #each, but experimentation in the REPL shows that it can take any sort of array literal, any sort of string, or a single identifier. (There may be more valid input, but someone else can explore those edge cases.)

String literals were already valid, so it was just a matter of consuming everything between balanced [ and ]. (Just for #each, though; array literals are not valid in #await.)

Originally I had it so that the calls to scan_for_balanced_character would inspect the returned boolean and return false themselves if they got a false value — i.e., bailing early from a catastrophic parsing failure instead of proceeding to the end of the file. But this broke the test case in random.txt, so I reverted that change.