Add new_from_data for FixedDecimalFormat

[Manishearth]

Progress on #560

@sffc the constructor for DateTimeFormat is a bit more complicated, what would you like its constructor to look like?

Also, you've mentioned in the past that we should make PluralRulesList the data format for PluralRules. Anything I should know to start doing this?

[coveralls]

Pull Request Test Coverage Report for Build 37b7a9f1af39432b6fa8cbadae94a4cdd9519cba-PR-933

• 0 of 4 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.03%) to 75.061%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
components/decimal/src/lib.rs	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
experimental/provider_ppucd/src/parse_ppucd.rs	1	93.28%

Totals
Change from base Build ae65821858b61560d984c28e2c2153ae39e087e2:	-0.03%
Covered Lines:	9535
Relevant Lines:	12703

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💛 - Coveralls