ICU-20568 Support smart units / unit contexts / preferences

ICU-20568 Support smart units / unit contexts / preferences

Explore Usage-related error codes, address icu-units/icu#36.
PR: https://github.com/icu-units/icu/pull/56
Commit: d5d7fdccfef887bb1af180bba3e2a0286dc32135

Implement Precision handling in UsagePrefsHandler::processQuantity
PR: https://github.com/icu-units/icu/pull/61
Commit: 16547f32986600a46e4adf20a6870c1708dd1c75

Support Mixed Units in NumberFormatter when using usage()
PR: https://github.com/icu-units/icu/pull/52
Commit: cc5a12202133855e15ffba889acffc10aad2d46b

For MixedUnits, use the correct ListFormatter styles.
PR: https://github.com/icu-units/icu/pull/66
Commit: 77bb747002d36626386f18e45c68c44b276cf575

Read the CLDR testData test files from the new location.
PR: https://github.com/icu-units/icu/pull/68
Commit: 6eb992e2b3c0bbe4870b554a5aa855b3636566d4

Fix double-precision maths in unit conversions
PR: https://github.com/icu-units/icu/pull/71
Commit: 78e88fbddef0f6817654d58c9c5dfeb6606324b9

Support .unit(MIXED_UNIT) without .usage(...).
PR: https://github.com/icu-units/icu/pull/72
Commit: 56ac7959375b8c9363ff022185165e52490c6c00

More commits:
- Reorder numbertest_api.cpp tests for consistent order.
- NumberFormatterApiTest: fold unitPipeline() into unitCompoundMeasure()
- Add some 'template class' instantiations for MSVC.
- Make trimField handle all whitespace, improve test messages
- Drop templated 'appendAll': it requires copy constructor
- Add protected MaybeStackArray::copyFrom()
- Add TODO(icu-units#67) and commented-out test case: use kUndefinedField for now
- Provide correct output order for units like "inch-and-foot"
- MSVC: export MaybeStackVector<MeasureUnit>
- Code review feedback: dependencies.txt and doc comments
- Consistent naming for code files: units_*