Move the icu crate to no_std

[Manishearth]

This fixes #812 and gates CI on it building for no-std. Further dependencies being pulled into icu will have to support no_std (or be conditional on the "std" feature)

cc @nciric

[coveralls]

Pull Request Test Coverage Report for Build 143beae85b82774086c0452323a395638eb87798-PR-911

• 4 of 9 (44.44%) changed or added relevant lines in 4 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 74.42%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
components/datetime/src/error.rs	1	2	50.0%
components/datetime/src/date.rs	1	3	33.33%
components/datetime/src/skeleton.rs	1	3	33.33%

Files with Coverage Reduction	New Missed Lines	%
components/datetime/src/fields/symbols.rs	1	69.35%

Totals
Change from base Build e97c8c3b3d9c129bbf63ffc8162080dd01931ee1:	0.0%
Covered Lines:	9310
Relevant Lines:	12510

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💛 - Coveralls

[sffc]

I'm impressed that this works fairly painlessly!