Normalize UTC to UTC

[Manishearth]

It seems like there are tests that depend on UTC being normalized to UTC and not Etc/UTC.

Spec text:

ECMAScript implementations must support an available named time zone with the identifier "UTC", which must be the primary time zone identifier for the UTC time zone. In addition, implementations may support any number of other available named time zones.

Broken by #460.

Another fix would be to change this in normalize_identifier_with_compiled

[Manishearth]

@sffc also pointed out that "ECMA canonical" requires a 2 year waiting period before a time zone is considered canonical.

As such we might wish to make this code generic so that we can maintain a list of canonical tz overrides.

[nekevss]

requires a 2 year waiting period before a time zone is considered canonical.

Ah, shoot. I recall reading that somewhere at some point, but it totally slipped my mind.

[sffc]

icu_time::zone::iana (specifically IanaParser) contains data and abstractions intended to implement the ECMA spec. It was my expectation that temporal_rs would utilize it.

https://unicode-org.github.io/icu4x/rustdoc/icu/time/zone/struct.IanaParser.html

https://unicode-org.github.io/icu4x/rustdoc/icu/time/zone/iana/index.html

For some use cases, the IanaParserExtendedBorrowed::iter_all function may be useful:

https://unicode-org.github.io/icu4x/rustdoc/icu/time/zone/iana/struct.IanaParserExtendedBorrowed.html#method.iter