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 avoids importing all of `BitVec.Lemmas` and `BitVec.BitBlast`
into `UInt.Lemmas`. (They are still imported into `SInt.Lemmas`; this
seems much harder to avoid.)
