The IP and socket address types all had `FromStr` implemented but the
implementations were not marked stable, nor was the error type returned ready to
be properly stabilized.
This commit marks the implementations of `FromStr` as stable and also renamed
the `ParseError` structure to `AddrParseError`. The error is now also an opaque
structure that cannot be constructed outside the standard library.
cc #22949
[breaking-change]
3e433738fb