core: Inherit possible string functionality

This moves as much allocation as possible from teh std::str module into
core::str. This includes essentially all non-allocating functionality, mostly
iterators and slicing and such.

This primarily splits the Str trait into only having the as_slice() method,
adding a new StrAllocating trait to std::str which contains the relevant new
allocation methods. This is a breaking change if any of the methods of "trait
Str" were overriden. The old functionality can be restored by implementing both
the Str and StrAllocating traits.

[breaking-change]

src/libstd/io/process.rs

@@ -428,7 +428,6 @@ impl Drop for Process {
 mod tests {
     use io::process::{ProcessConfig, Process};
     use prelude::*;
-    use str::StrSlice;
 
     // FIXME(#10380) these tests should not all be ignored on android.