Add dedicated impls for PluralOperands::from for integers, avoid unnecessary string allocations

[Manishearth]

Currently we always convert to a string before parsing. This has two problems:

• For &str inputs, we unconditionally copy and allocate despite never needing a String
• For integers, we spend extra time reparsing back to an integer

This uses a new trait so that we can specialize the implementation and be fast for the common cases.

part of https://github.com/unclenachoduh/pluralrules/issues/14

r? @unclenachoduh @zbraniecki

[coveralls]

Pull Request Test Coverage Report for Build 104

• 13 of 18 (72.22%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 89.85%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
intl_pluralrules/src/operands.rs	12	17	70.59%
<!--	Total:	13	18	72.22%	-->

Totals
Change from base Build 103:	-0.5%
Covered Lines:	540
Relevant Lines:	601

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💛 - Coveralls