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/error.rs

@@ -192,7 +192,7 @@ impl Error + 'static {
                 let to: TraitObject = transmute(self);
 
                 // Extract the data pointer
-                Some(transmute(to.data))
+                Some(&*(to.data as *const T))
             }
         } else {
             None
@@ -210,7 +210,7 @@ impl Error + 'static {
                 let to: TraitObject = transmute(self);
 
                 // Extract the data pointer
-                Some(transmute(to.data))
+                Some(&mut *(to.data as *const T as *mut T))
             }
         } else {
             None