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/librustdoc/test.rs

@@ -300,7 +300,7 @@ pub fn maketest(s: &str, cratename: Option<&str>, dont_insert_main: bool,
 }
 
 fn partition_source(s: &str) -> (String, String) {
-    use unicode::str::UnicodeStr;
+    use rustc_unicode::str::UnicodeStr;
 
     let mut after_header = false;
     let mut before = String::new();