net/netip: add Addr.AsSlice() method

We have AddrFrom4, AddrFrom6, AddrFromSlice and As4, As6, but we are
missing AsSlice, so this commit adds the missing function. It also gets
rid of the less ergonomic and inconsistently named IPAddrParts.

Updates #49298.

Change-Id: I1c6a2c32fc6c69b244ab49765412ffe3bbe7e5c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/360874


Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>