Replace many uses of mem::transmute with more specific functions

The replacements are functions that usually use a single `mem::transmute` in
their body and restrict input and output via more concrete types than `T` and
`U`. Worth noting are the `transmute` functions for slices and the `from_utf8*`
family for mutable slices. Additionally, `mem::transmute` was often used for
casting raw pointers, when you can already cast raw pointers just fine with
`as`.

src/libstd/ascii.rs

@@ -15,7 +15,6 @@
 use prelude::v1::*;
 
 use ops::Range;
-use mem;
 
 /// Extension methods for ASCII-subset only operations on owned strings
 #[unstable(feature = "owned_ascii_ext",
@@ -186,12 +185,12 @@ impl AsciiExt for str {
     }
 
     fn make_ascii_uppercase(&mut self) {
-        let me: &mut [u8] = unsafe { mem::transmute(self) };
+        let me: &mut [u8] = unsafe { self.as_bytes_mut() };
         me.make_ascii_uppercase()
     }
 
     fn make_ascii_lowercase(&mut self) {
-        let me: &mut [u8] = unsafe { mem::transmute(self) };
+        let me: &mut [u8] = unsafe { self.as_bytes_mut() };
         me.make_ascii_lowercase()
     }
 }