Io.net: expand IPv6:IPv4 formatting and parsing

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The IPv6 formatter code now emits the :IPv4 alternate text form for both "IPv4 Mapped" and "Well-Known Prefix" (RFC 6052).

The IPv6 parser now accepts the :IPv4 form regardless of what the first 12 bytes are, so long as there is no trailing "%interface" (which should only be useful for link-locals anyways). RFC 4291 does not require any special prefix for the :IPv4 form on the parsing side, and there are many non-canonical cases in the real world which should still parse correctly.