Req. generic masking shifts

[oscbyspro]

I need to overload Swift's generic masking shifts to fix an issue in the standard library's implementation for non-power-of-two bit width integers. But, that said, I want to keep the overloads to a minimum. Adding requirements to ANKFixedWidthInteger, with a custom implementation in ANKFullWidth, makes it so core integers continue to use the standard library's implementation while ANKFullWidth receives the fix. Using ANKFullWidth in the context of FixedWidthInteger will result in the standard library's behavior.

static func &<<=(lhs: inout Self, rhs: some BinaryInteger)
static func &<< (lhs: Self, rhs:  some BinaryInteger) -> Self
static func &>>=(lhs: inout Self, rhs: some BinaryInteger)
static func &>> (lhs: Self, rhs:  some BinaryInteger) -> Self

[oscbyspro]

I added protocol witness tests for ANKFixedWidthInteger in 762a850. Removing the aforementioned requirements will therefore cause some test cases to fail. If removing the requirements does not cause any test cases to fail, then  has fixed Swift's standard library implementation.