linalg/bitarray: bugfixes (istril/u, kron)

Also add tests
bitarray: implement preprend! and sizehint
bitarray: make bitshift operators saturating

This is for consistency with how those operators are
implemented for numbers.

Also, code cosmetics.
linalg/bitarray: additional check in dot

Also, code cosmetics
bitarray: remove some bound checks

Use inbounds macro where it makes a noticeable
difference (more than 10% performance increase).
bitarray: small improvement in kron