Implement syntax-constructor ("scons") mechanism.

Implement syntax-constructor ("scons") mechanism.

This eliminates a potential problem with RSC and ER macros, which typically
construct ordinary list structure using quasiquote and renaming the keywords.
Unfortunately this will fail if the use environment has redefined the quasiquote
and/or quote keywords.  These constructors are careful not to use any keywords
except renamed ones; they also hide most of the renaming while providing a
simple procedural interface.