Auto merge of #23504 - alexcrichton:parse-error-not-unit, r=aturon

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]