[plt-scheme] hygienic macros

I realize this was just an exercise in converting defmacro to
define-syntax, but isn't `case' the wrong thing here?  PLT Scheme just
expands it to a big nested `if' expression; I think it would be better
to convert the expression list into a vector of thunks.  But maybe
the added time (and memory) requirements for all that procedure
construction would overwhelm the O(n) tests in the `case'
expression...  I don't know enough about PLT internals to know whether
the compiler could optimize one better than the other.

--dougo at place.org