PRNG state UInt->Vec[Bool], make async reset safe

Changes the internal state of PRNG to use Vec[Bool] instead of UInt.
This fixes an @aswaterman identified future problem with asynchronous
reset.

A register with an asynchronous reset can only be reset to a literal.
Previously, an LFSR would store state as a UInt. If it was not
parameterized with a seed it should have its least significant bit
reset to something to avoid locking up. It's ideal to not reset the
full UInt (better test coverage, decreased reset fanout). However,
it's difficult to only reset one bit of a UInt. Conversely, it's
trivial to reset one bit of a Vec[Bool]. This also moves PRNG/LFSR
closer to a canonical representation of their internal state, i.e.,
it's natural to think of generalizing Vec[Bool] to arbitrary finite
fields (Vec[A <: Field]) whereas UInt is tightly coupled to GF2.

Minor updates:

- Updates/fixes to some scaladoc
- Add assertion to period test to make sure LFSR is changing

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>