Stabilize some Result methods as const

Stabilize the following methods of `Result` as const:
 - `is_ok`
 - `is_err`
 - `as_ref`

Possible because of stabilization of #49146 (Allow if and match in constants).

library/core/src/lib.rs

@@ -87,7 +87,6 @@
 #![feature(const_ptr_offset)]
 #![feature(const_ptr_offset_from)]
 #![feature(const_raw_ptr_comparison)]
-#![feature(const_result)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_slice_ptr_len)]
 #![feature(const_size_of_val)]