IFS incompatibilities

[andychu]

There are a bunch of related bugs, let's catgorize them here:

• Can't escape closing brace with backslash or single quotes in parameter expansion #702 -- Can't escape closing brace with backslash or single quotes in parameter expansion -- not sure what this one is.
  • failing test cases: spec/var-op-strip
• single quotes within double quoted brace sub treated differently for the # ## % %% / operators #291 -- single quotes within double quoted brace sub treated differently for the # ## % %% operators --- might be related to the above, not sure
  • failing test cases: spec/var-sub-quote

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split and glob:

• Error with backslashes in unquoted variables with globbing off #698 -- set -o noglob, echo $var does backslash escaping when it shouldn't.
  • this is definitely in osh/word_eval.py
  • failing test cases: spec/glob
• Incorrect behavior of $@ and $* when IFS='' #627 -- Incorrect behavior of $@ and $ when IFS='' *
  • just like no globbing, we also have problems when not splitting
  • failing test cases: spec/word-split
  • Fix POSIX compliance issues uncovered by smoosh (Run smoosh test suite and make a report) #331 -- smoosh also reports an empty IFS bug, not sure if it's the same
• IFS=: and : in literal word gives incorrect results #628 -- IFS=':' and : in literal word. It no longer crashes, but it gives incorrect results

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Architectural issue: #604 consider an IR for ${}