Provides stable const implementations for some things missing from the standard library.

Currently implemented are

• Functions in result to unwrap Results with generics or drop glue
• Functions in the concat module to concat const strings and byte slices.
• destruct_tuple to destructure tuples with generic types or types with drop glue in them
• nonnull_from to create NonNulls from mutable and regular references conveniently
• man_drop_ref/man_drop_mut as a workaround for the lack of const Deref implementations