Thanks, Lassi, for your newest SRFI.

- If numbers are unreliable, don't expose them to the user but wrap
them in opaque objects.  Only necessarily system-dependent low-level
implementations of libraries should deal with them.

- Use macros instead of procedures to catch typos and use unsupported
symbols early.  In particular, write (codeset errno) instead of
'errno.  Write (errno eperm) instead of 'eperm.  (See R6RS
enumerations that use the same technique, for example.)