Added bvec constructors

[vallentin]

I noticed that all BVec*s are missing the freestanding fn *vec*() constructors.

[vallentin]

The lint was triggered by a missing cfg attr for scalar-math. Fixed it and rebase.

[bitshifter]

Looks good, I have one small request, could you add some test functions? While these are trivial functions it's useful to have tests for code coverage (to see what still needs testing) and also if I ever refactor to make sure I don't miss some functionality.

If you look in tests/vec*.rs there are tests that start with test_mask_, you could either add them to an existing test or add a new one that does something like assert_eq!($mask::new(false, false), $bvec2(false, false))

One catch is you would need to add $bvec2 to the macro signature and pass it through.