add str::SplitInclusive::as_str method

This commit entroduces `core::str::SplitInclusive::as_str` method similar to
`core::str::Split::as_str`, but under different gate -
"str_split_inclusive_as_str" (this is done so because `SplitInclusive` is
itself unstable).

library/core/src/lib.rs

@@ -126,6 +126,7 @@
 #![feature(std_internals)]
 #![feature(stmt_expr_attributes)]
 #![feature(str_split_as_str)]
+#![feature(str_split_inclusive_as_str)]
 #![feature(transparent_unions)]
 #![feature(unboxed_closures)]
 #![feature(unsized_locals)]