[Foundation] Collapse (SignedInteger|UnsignedInteger) reqts into FixedWidthInteger.
[Foundation] Collapse (SignedInteger|UnsignedInteger) reqts into FixedWidthInteger.

Simplify the default implementations provided for RawRepresentable
error types whose raw values are integral, replacing duplicated code
(for SignedInteger or UnsignedInteger) with a single constraint on
BinaryInteger.

Fixes rdar://problem/35230187.

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation