add width() method in Char ; update Unicode tables

- Updated `src/etc/unicode.py` to generate a table for determining
  character width.
- Miscellaneous updates in `src/etc/unicode.py` to correspond to hand-
  written updates applied to `src/libcollections/unicode.rs` earlier this year.
- Generated updated versions of `src/libcore/unicode.rs` and
  `src/libcollections/unicode.rs` based on the latest Unicode data.
- Added new `width` method and corresponding implementation in the
  `Char` trait.
- Added basic test for `.width()` in libcoretest.

closes #15224