Add Public trait to create public FIRRTL modules (#3813)

Add a new trait, `Public`, which can be used to emit FIRRTL modules which
have the public keyword.  This is a weak form of "public modules" that we
want to support for FIRRTL 4.0.0.  This does not allow for multiple,
disjoint instance graphs to be contained in a circuit, nor does it remove
the "main module" from the circuit.  However, it provides a mechanism for
plucking modules out of a circuit later and working with them.

This is self-typed on `RawModule` to prevent mix-in to an external module.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>