std: Tweak some unstable features of str

This commit clarifies some of the unstable features in the `str` module by
moving them out of the blanket `core` and `collections` features.

The following methods were moved to the `str_char` feature which generally
encompasses decoding specific characters from a `str` and dealing with the
result. It is unclear if any of these methods need to be stabilized for 1.0 and
the most conservative route for now is to continue providing them but to leave
them as unstable under a more specific name.

* `is_char_boundary`
* `char_at`
* `char_range_at`
* `char_at_reverse`
* `char_range_at_reverse`
* `slice_shift_char`

The following methods were moved into the generic `unicode` feature as they are
specifically enabled by the `unicode` crate itself.

* `nfd_chars`
* `nfkd_chars`
* `nfc_chars`
* `graphemes`
* `grapheme_indices`
* `width`

src/libcore/str/mod.rs

@@ -18,6 +18,7 @@
 
 use self::OldSearcher::{TwoWay, TwoWayLong};
 
+use char::CharExt;
 use clone::Clone;
 use cmp::{self, Eq};
 use default::Default;
@@ -1112,8 +1113,10 @@ static UTF8_CHAR_WIDTH: [u8; 256] = [
 /// the next `char` in a string.  This can be used as a data structure
 /// for iterating over the UTF-8 bytes of a string.
 #[derive(Copy)]
-#[unstable(feature = "core",
-           reason = "naming is uncertain with container conventions")]
+#[unstable(feature = "str_char",
+           reason = "existence of this struct is uncertain as it is frequently \
+                     able to be replaced with char.len_utf8() and/or \
+                     char/char_indices iterators")]
 pub struct CharRange {
     /// Current `char`
     pub ch: char,
@@ -1646,8 +1649,8 @@ impl StrExt for str {
         if self.is_empty() {
             None
         } else {
-            let CharRange {ch, next} = self.char_range_at(0);
-            let next_s = unsafe { self.slice_unchecked(next, self.len()) };
+            let ch = self.char_at(0);
+            let next_s = unsafe { self.slice_unchecked(ch.len_utf8(), self.len()) };
             Some((ch, next_s))
         }
     }