Added const versions of common numeric operations

# Conflicts:
#	library/core/src/lib.rs

library/core/src/lib.rs

@@ -116,6 +116,7 @@
 #![feature(const_maybe_uninit_as_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_num_from_num)]
+#![feature(const_ops)]
 #![feature(const_option)]
 #![feature(const_pin)]
 #![feature(const_replace)]