If you set the macro-aux of a macro outside of (chibi syntax-case), it
would previously case `syntax` to think that it was a pattern variable
and try to substitute it, even if the macro-aux was being used for
something else.
This patch fixes that by wrapping pattern variable values in an extra
typed box and checking that it has the right type before deciding that
it’s actually a pattern variable.
This restores third-party (ab)users of the Chibi macro system such
as in https://gist.github.com/baguette/2632464, while allowing us
to break those uses in more interesting ways.
It also keeps the core slightly smaller (both in C and Scheme)
and speeds up the macro expansion process.
