Prelude: rename and consolidate extension traits

This commit renames a number of extension traits for slices and string
slices, now that they have been refactored for DST. In many cases,
multiple extension traits could now be consolidated. Further
consolidation will be possible with generalized where clauses.

The renamings are consistent with the [new `-Prelude`
suffix](https://github.com/rust-lang/rfcs/pull/344). There are probably
a few more candidates for being renamed this way, but that is left for
API stabilization of the relevant modules.

Because this renames traits, it is a:

[breaking-change]

However, I do not expect any code that currently uses the standard
library to actually break.

Closes #17917

src/libstd/ascii.rs

@@ -19,8 +19,8 @@ use fmt;
 use iter::Iterator;
 use mem;
 use option::{Option, Some, None};
-use slice::{ImmutableSlice, MutableSlice, AsSlice};
-use str::{Str, StrSlice};
+use slice::{SlicePrelude, AsSlice};
+use str::{Str, StrPrelude};
 use string::{mod, String};
 use to_string::IntoStr;
 use vec::Vec;
@@ -578,7 +578,7 @@ mod tests {
     use prelude::*;
     use super::*;
     use char::from_u32;
-    use str::StrSlice;
+    use str::StrPrelude;
 
     macro_rules! v2ascii (
         ( [$($e:expr),*]) => (&[$(Ascii{chr:$e}),*]);