[CM] R7RS support

[bil at ccrma.Stanford.EDU]

On the guile library filename handling, does any other scheme
treat the names that way?  I have added the rest of the built-in
libaries to r7rs.scm: eval, process-context, case-lambda, lazy,
load, and repl.  s7 ignores these import statements.


(display "hi" (current-error-port))

works for me.  It is the same as (display "hi" *stderr*); since
there is no buffering in this case, the flush-output-port has
no effect, unless someone has redirected stderr.