* The langspec definition of `@memcpy` has been changed so that the
source and destination element types must be in-memory coercible,
allowing all such calls to be raw copying operations, not actually
applying any coercions.
* Implement aliasing check for comptime `@memcpy`; a compile error will
now be emitted if the arguments alias.
* Implement more efficient comptime `@memcpy` by loading and storing a
whole array at once, similar to how `@memset` is implemented.
