Add kind "KBool" to let us distinguish SMTLib types Bool and BitVec 1.

This avoids generation of non-type-correct SMTLib output for proofs like:
 prove $ \x -> (x::SWord1) `shiftL` 1 .== 2*x
Merge branch 'master' of https://github.com/brianhuffman/sbv into brianhuffman-master
careful with zero with KBool kind
Merge branch 'brianhuffman-master'
Minor tweaks to support KBool
mention KBool in CHANGES