Organize properties code to be more like the other crates (get rid of free functions)

[Manishearth]

This is probably a 2.0 change (https://github.com/unicode-org/icu4x/issues/2856)

The properties code uses a lot of free functions for loading property maps and such, rather than using constructors on CodePointSetData and CodePointMapData as would be tradition in the other crates.

This makes things a bit awkward. We should consider this design again for 2.0.

Some open questions:

• How do we deal with the fact that CodePointMapData is generic, and that Rust users will be using CodePointMapData<GeneralCategory> (etc) whereas FFI uses an erased version?
• How do we expose default baked data in this regime?

To-do items:

• icu_properties::exemplar_chars fns should not need "exemplar" in the name
• The mapper functions should be named load_ instead of get_

cc @robertbastian @sffc

Low-priority discussion since it's likely a 2.0 issue.