[utils] Drop codepoints module

[behnam]

• We have developed unic-char-range module to replace iter_all_chars(), and there no more functionality left in unic-utils's codepoints module, therefore dropping the module completely.

• Replace all iter_all_chars() calls with CharRange::all().

• It's common practice to start the name of all test functions (those with #[test] attr) with test_, to help differentiating them from other functions. We follow it for most cases, but were missing it in some.

• Add copyright notice to files missing it.

[behnam]

bors: r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push