Parsers should support unvalidated UTF16 and Latin-1

[Manishearth]

Currnetly the parsers, like Instant::from_str() requires validated UTF8 str.

JS engines typically have UTF16 or unvalidated UTF8.

It would be nice if these parsers were written to consume &[u8] (most of the parsing is ASCII only anyway), so we could at least operate on unvalidated UTF8 and have from_utf8_bytes() functions.

Ideally we also have UTF16 functions. That would need a tweak to the ixdtf parser.

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Update: This should also handle Latin-1. Our parse functions mostly just care about ASCII anyway so it's not a big deal, but it is somewhat better from a cleanliness pov to accept it.

[Manishearth]

cc @nekevss

[Manishearth]

A thing I may do over capi is adding from_str_utf8() from_str_utf16() that take DiplomatStr and DiplomatStr16, converting internally. Then over time we can make optimizations to avoid conversions/checking.

[HalidOdat]

Parsing unvalidated utf8 has been implemented in #295

[nekevss]

With unicode-org/icu4x#6577 merged, implementing UTF16 support should mostly be unblocked.