'(_hi_)' was being parsed with literal underscores (no emphasis).
The fix: the 'str' parser now only parses alphanumerics and
embedded underscores. All other symbols are handled by the
'symbol' parser. This has a slight effect on the AST, since
you'll get [Str "hi",Str ":"] insntead of [Str "hi:"]. But there
should not be a visible effect in any of the writers.
Thanks to gwern for pointing out the regression.
John MacFarlane
3e61333af0
