This PR adjusts the experimental module system to make `private` the
default visibility modifier in `module`s, introducing `public` as a new
modifier instead. `public section` can be used to revert the default for
an entire section, though this is more intended to ease gradual adoption
of the new semantics such as in `Init` (and soon `Std`) where they
should be replaced by a future decl-by-decl re-review of visibilities.
This PR adds support for the following import variants to the
experimental module system:
* `private import`: Makes the imported constants available only in
non-exported contexts such as proofs. In particular, the import will not
be loaded, or required to exist at all, when the current module is
imported into other modules.
* `import all`: Makes non-exported information such as proofs of the
imported module available in non-exported contexts in the current
module. Main purpose is to allow for reasoning about imported
definitions when they would otherwise be opaque. TODO: adjust name
resolution so that imported `private` decls are accessible through
syntax.
They can be combined into `private import all`, which will likely be the
most common usage of `import all`.
