Comment typo fix
Made list-set! built-in
Added boolean=? and string=? to core
Support #true and #false

This changes the edge cases of delimiting rules and tokens including '#'.
See the ChangeLog for the details.
Fixes in r7rs.scm and example
Let r7rs-import check if module is already on memory

... and avoid requiring the file if it is.  It allows the programmer
to define-library without having an actual file.
Change 'use' according to the change of 'require'

Now 'require' guarantees to load into gauche module, we no longer
need the with-module hack; in fact, the with-module hack hadn't been
working as intended for some time.
Fixed leak in thread-sleep! and port
Cosmetic changes