* boot-9.scm (define-public): Removed spurious call to

* boot-9.scm (define-public): Removed spurious call to
`interaction-evironment'.
(define-public, defmacro-public): Use `export' instead of explicit
module magic.
(eval-when): New macro.
(define-module, use-modules, use-syntax, export): Use it to
restrict the use of these forms to the top level.
(define-public, defmacro-public): Only export binding when on
top-level.
(process-define-module): Call `set-current-module' with the
defined module.
(define-module): Simply call `process-define-module' without any
fuss (but only on top-level).
(named-module-use!): New function.
(top-repl): Do not use `define-module'.  Use equivalent low-level
means instead.