Fixed bugs related to define/generic and to define-generics at top-level.

Added tests using define/generic in both #:defaults and #:fallbacks, both inside
modules and at the top level.  Changed the implementation of define-generics so
that defaults and fallbacks come after method definitions so that define/generic
works properly.  Changed internal names to be created with format-id and
syntax-local-introduce rather than generate-temporaries so that they work at the
top level.