auto merge of #15283 : kwantam/rust/master, r=alexcrichton

Add libunicode; move unicode functions from core

- created new crate, libunicode, below libstd
- split `Char` trait into `Char` (libcore) and `UnicodeChar` (libunicode)
  - Unicode-aware functions now live in libunicode
    - `is_alphabetic`, `is_XID_start`, `is_XID_continue`, `is_lowercase`,
      `is_uppercase`, `is_whitespace`, `is_alphanumeric`, `is_control`, `is_digit`,
      `to_uppercase`, `to_lowercase`
  - added `width` method in UnicodeChar trait
    - determines printed width of character in columns, or None if it is a non-NULL control character
    - takes a boolean argument indicating whether the present context is CJK or not (characters with 'A'mbiguous widths are double-wide in CJK contexts, single-wide otherwise)
- split `StrSlice` into `StrSlice` (libcore) and `UnicodeStrSlice` (libunicode)
  - functionality formerly in `StrSlice` that relied upon Unicode functionality from `Char` is now in `UnicodeStrSlice`
    - `words`, `is_whitespace`, `is_alphanumeric`, `trim`, `trim_left`, `trim_right`
  - also moved `Words` type alias into libunicode because `words` method is in `UnicodeStrSlice`
- unified Unicode tables from libcollections, libcore, and libregex into libunicode
- updated `unicode.py` in `src/etc` to generate aforementioned tables
- generated new tables based on latest Unicode data
- added `UnicodeChar` and `UnicodeStrSlice` traits to prelude
- libunicode is now the collection point for the `std::char` module, combining the libunicode functionality with the `Char` functionality from libcore
  - thus, moved doc comment for `char` from `core::char` to `unicode::char`
- libcollections remains the collection point for `std::str`

The Unicode-aware functions that previously lived in the `Char` and `StrSlice` traits are no longer available to programs that only use libcore. To regain use of these methods, include the libunicode crate and `use` the `UnicodeChar` and/or `UnicodeStrSlice` traits:

    extern crate unicode;
    use unicode::UnicodeChar;
    use unicode::UnicodeStrSlice;
    use unicode::Words; // if you want to use the words() method

NOTE: this does *not* impact programs that use libstd, since UnicodeChar and UnicodeStrSlice have been added to the prelude.

closes #15224
[breaking-change]

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