Remove unused AsciiExt imports and fix tests related to ascii methods

Many AsciiExt imports have become useless thanks to the inherent ascii
methods added in the last commits. These were removed. In some places, I
fully specified the ascii method being called to enforce usage of the
AsciiExt trait. Note that some imports are not removed but tagged with
a `#[cfg(stage0)]` attribute. This is necessary, because certain ascii
methods are not yet available in stage0. All those imports will be
removed later.

Additionally, failing tests were fixed. The test suite should exit
successfully now.

src/liballoc/str.rs

@@ -390,8 +390,6 @@ impl str {
     /// # Examples
     ///
     /// ```
-    /// use std::ascii::AsciiExt;
-    ///
     /// let mut v = String::from("hello");
     /// // correct length
     /// assert!(v.get_mut(0..5).is_some());
@@ -617,8 +615,6 @@ impl str {
     /// Basic usage:
     ///
     /// ```
-    /// use std::ascii::AsciiExt;
-    ///
     /// let mut s = "Per Martin-Löf".to_string();
     /// {
     ///     let (first, last) = s.split_at_mut(3);