add str::{Split,RSplit}::as_str methods

This commit introduses 2 methods under "str_split_as_str" gate with common
signature of `&Split<'a, _> -> &'a str'`. Both of them work like
`Chars::as_str` - return unyield part of the inner string.

library/core/src/lib.rs

@@ -125,6 +125,7 @@
 #![feature(staged_api)]
 #![feature(std_internals)]
 #![feature(stmt_expr_attributes)]
+#![feature(str_split_as_str)]
 #![feature(transparent_unions)]
 #![feature(unboxed_closures)]
 #![feature(unsized_locals)]