ICU-20568 Implement Usage "Glue Code" and skeleton support. Tests.

ICU-20568 Implement Usage "Glue Code" and skeleton support. Tests.

CLDR-13488: Samples of intermediate source/data/misc/units.txt
PR: https://github.com/icu-units/icu/pull/5
Commit: 08132e7a3388e055590e5ba60ba37c9b5a87b900

Unit test improvements (the rest of PR already upstreamed).
PR: https://github.com/sffc/icu/pull/40
Commit: e92a2fcffbbcb92868954f28e3366568e6b3cc7c

Use namespace ::icu::units::impl for units-related code.
PR: https://github.com/icu-units/icu/pull/16
Commit: 57b35a8a93e751aa73a190e63da494d24fd67bef

Export addSingleFactorConstant in header file
PR: https://github.com/icu-units/icu/pull/17
Commit: f357b5558ebc2f0aab53886492beae7f17d9c3cc

Fix MeasureUnit identifiers to be spec-compliant.
PR: https://github.com/icu-units/icu/pull/18
Commit: 69c087154b871b53fdba978bfa1b1ba4c4e71594

Cleanup: IWYU headers (with some exceptions), SigNum -> Signum
PR: https://github.com/icu-units/icu/pull/19
Commit: 1c29a6bcc74c113a7d51b008a93cd7986437c841

Test that addSingleFactorConstant knowns all unitConstants.
PR: https://github.com/icu-units/icu/pull/20
Commit: 6c0c3bb64080354ea109d1356b98ac7a3a97f443

Add Unit Usage support to Number Skeletons.
PR: https://github.com/icu-units/icu/pull/33
Commit: 3cf23222ca8c7fa394e3ed37ec8333f5e86c6807

Fix some compiler warnings
PR: https://github.com/icu-units/icu/pull/43
Commit: be11edb0bd70a6857ea24e7427725b803263481d