replace cenv with things in <repl> and fluids; remove the `use' meta-command

replace cenv with things in <repl> and fluids; remove the `use' meta-command

* module/system/base/compile.scm (<cenv>): No more cenv, it was a useless
  data structure.

* module/system/repl/command.scm (*command-table*): Remove `use', it's
  the same as `import'. Otherwise in this file, adapt to the repl having
  direct pointers to the vm and the language, and to the module being in
  the current-module fluid.

* module/system/repl/repl.scm (prompting-meta-read):
* module/system/repl/common.scm (<repl>): The repl now has a direct
  pointer to the vm and language. Adapt accordingly.