Add the `-warn-implicit-overrides` flag when building the standard library
and overlays, so that each protocol member that overrides a member of an
inherited protocol will produce a warning unless annotated with either
‘override’ or ‘@_nonoverride’.
An annotation of `override` will mean that the overriding requirement will be treated identically to the overridden declaration. If for some reason a concrete type’s conformance to the inheriting protocol provides a different witness for the overriding requirement than the conformance to the inherited protocol’s witness for the overridden requirement, the witness for the inheriting (more-specialized) protocol will be ignored. A protocol requirement marked ‘override’ only makes sense when the declaration is needed to help associated type inference, which is why the ‘override’ annotations correlate so closely with ABI FIXMEs.
An annotation of `@_nonoverride` means that the two protocol requirements will be treated independently, and may be bound to different witnesses. Use `@_nonoverride` when we might need different witnesses, e.g., because the semantics of the potentially-overriding declaration differ from that of the potentially-overridden declaration. `BidirectionalCollection.index(_:offsetBy:)` is the most obvious example, because the `BidirectionalCollection` ’s version of `index(_:offsetBy:)` allows negative indices. `RandomAccessCollection` ’s version is also marked `@_nonoverride` because it is required to be asymptotically faster than the `Collection` or `BidirectionalCollection` versions.
In order to provide source compatibility with existing user types conforming to BinaryInteger, we want to have a default implementation available. It's somewhat difficult to provide a good default implementation that correctly handles arbitrary non-symmetrical ranges in the face of negative divisors, so fall back on testing divisibility of the magnitudes, which avoids the problem.
On the plus side, this default implementation works fine for types conforming to UnsignedInteger, which lets us move the FixedWidthInteger implementation down to FixedWidthInteger & SignedInteger, and simplify it in the process.
* Implement SE-0225 (BinaryInteger.isMultiple(of:))
A default implementation is provided for FixedWidthInteger, with very basic test coverage included.
