core: introduce split_at{,_mut}_checked

Introduce split_at_checked and split_at_mut_checked methods to slices
types (including str) which are non-panicking versions of split_at and
split_at_mut  respectively.  This is analogous to get method being
non-panicking version of indexing.

library/core/src/lib.rs

@@ -184,6 +184,7 @@
 #![feature(set_ptr_value)]
 #![feature(slice_ptr_get)]
 #![feature(slice_split_at_unchecked)]
+#![feature(split_at_checked)]
 #![feature(str_internals)]
 #![feature(str_split_inclusive_remainder)]
 #![feature(str_split_remainder)]