a SRFI 60 installed which imports (scheme base). This can still break
in theory if a user installs a third-party SRFI 33 in the search path
in front of the Chibi SRFI 33, but we can't always be safe against
such behavior.
Fixes issue #267.
Many still import (chibi), and as (scheme base) is somewhat more
expensive to load at present these are changed to cond-expand.
Many libraries also rely on (srfi 33), and these have been changed
to a cond-expand first trying (srfi 60) where available.
Also fixing a few portability concerns (duplicate imports of the
same binding), and adding a few libraries missing from lib-tests.scm.
