Add C FFI for LocaleCanonicalizer

[dminor]

Fix for #756.

[coveralls]

Pull Request Test Coverage Report for Build c990bac47a61538c841b0167033a085255750a01-PR-772

• 0 of 23 (0.0%) changed or added relevant lines in 2 files are covered.
• 114 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.5%) to 75.652%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
components/locale_canonicalizer/src/locale_canonicalizer.rs	0	4	0.0%
ffi/capi/src/locale_canonicalizer.rs	0	19	0.0%

Files with Coverage Reduction	New Missed Lines	%
components/locale_canonicalizer/src/locale_canonicalizer.rs	50	76.3%
experimental/segmenter/src/line_breaker.rs	64	82.39%

Totals
Change from base Build da5bfbfdfa8dc7946530ec3ec97b365e1ab627c7:	-0.5%
Covered Lines:	9374
Relevant Lines:	12391

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💛 - Coveralls

[zbraniecki]

comment (nb): reading this I realized that we hide the reasons constructor fails in C API. I hope we'll improve it one day since constructor failures are likely going to be most common in production environments.

[Manishearth]

looks pretty straightforward, glad it worked out to be mostly along the lines of the existing FFI!

[Manishearth]

issue: we should probably actually test that this matches expectations

(make sure the locale test is being run in Makefile.toml, btw, it's not at the moment)

[sffc]

comment (nb): reading this I realized that we hide the reasons constructor fails in C API. I hope we'll improve it one day since constructor failures are likely going to be most common in production environments.

Tracking issue, last discussed 2021-05-20 and assigned to @Manishearth for 0.4: #703