support meta-commands to remove variables and unimport (or reimport) modules

[timotheecour]

without unimport (or reimport), there's no way to modify a module once imported

[Tangdongle]

This could possibly be fixed by changing the scrFile argument to be compiled in addition to the main buffer code, as opposed to being inserted as preloaded data.

For the import calls, we could probably support unimport. Unimport would delete a line matching import <name> from the source buffer. Modules imported in the repl should be automatically recompiled if the import source code changes (I think, maybe it needs a -f to force compilation)