SC is as standard as possible (aiming mostly at R4RS), but features
are implemented as needed.  Gratuitous incompatibilities are bugs.

Exceptions to R4RS
   - characters are integers
   - strings are byte vectors
   - numbers: only fixnums (machine word size - 2 bits signed
     integers) and flonums (C's double)
   - only defmacro


Features planned:
   - proper characters
   - complex numbers
   - optional bignums