Fix a bug regarding export inherited bindings with renaming

When we saw renaming export during binding searching, we should
use the original name to go up MPL; but we've used the renamed name.
Another bug fix w/ export renaming, import renaming and phantom bindings.
Remove temporary code regarding keyword in syntax-rule literals

The code was necessary to compile 0.9.4 with 0.9.3.3.  No longer needed.
Split gauche.base from r7rs.scm

gauche.base can be useful from Gauche code and it's better to
allow (use gauche.base).  See the document update for the
use case.