deprecate Unicode functions that will be moved to crates.io

This patch
1. renames libunicode to librustc_unicode,
2. deprecates several pieces of libunicode (see below), and
3. removes references to deprecated functions from
   librustc_driver and libsyntax. This may change pretty-printed
   output from these modules in cases involving wide or combining
   characters used in filenames, identifiers, etc.

The following functions are marked deprecated:

1. char.width() and str.width():
   --> use unicode-width crate

2. str.graphemes() and str.grapheme_indices():
   --> use unicode-segmentation crate

3. str.nfd_chars(), str.nfkd_chars(), str.nfc_chars(), str.nfkc_chars(),
   char.compose(), char.decompose_canonical(), char.decompose_compatible(),
   char.canonical_combining_class():
   --> use unicode-normalization crate

src/libcollectionstest/string.rs

@@ -135,7 +135,7 @@ fn test_from_utf16() {
         let s_as_utf16 = s.utf16_units().collect::<Vec<u16>>();
         let u_as_string = String::from_utf16(&u).unwrap();
 
-        assert!(::unicode::str::is_utf16(&u));
+        assert!(::rustc_unicode::str::is_utf16(&u));
         assert_eq!(s_as_utf16, u);
 
         assert_eq!(u_as_string, s);