New feature: unscoped module names

[klmr]

This PR is a WIP implementation of unscoped module names. For now, the syntax for specifying an unscoped module name xyz is box::use(mod(xyz)). mod(…) disambiguates between package and module names, and can be passed any module name, including scoped ones (both global and local, e.g. mod(foo/bar) and mod(./foo)).

The current placeholder syntax for disambiguation (i.e. mod(…)) is subject to change before the PR is merged. For suggestions, see discussion at #307.

For now I’ve narrowed this list down to the following candidates:

1. box::use(mod:module_name)
2. box::use(mod::module_name)

Feedback welcome.