[R6RS] library.tex edits

[dyb at cs.indiana.edu]

I've committed a new version of library.tex that eliminates the export
"for" syntax and specifies how export phases are determined.  I've
also added text to library.tex specifying the default export phases
of bindings exported from standard libraries, and added a note to
complib.tex mentioning that it exports all of its bindings for both
run and expand.

Matthew, since I'm getting this to you so late, I'll give you until
11:00pm EDT (an hour from now) to commit any fixes to my changes.

Kent