We have primops for copying ranges of bytes between ByteArray#s:
 * ByteArray# -> MutableByteArray#
 * MutableByteArray# -> MutableByteArray#
This extends it with three further cases:
 * Addr# -> MutableByteArray#
 * ByteArray# -> Addr#
 * MutableByteArray# -> Addr#
One use case for these is copying between ForeignPtr-based
representations and in-heap arrays (like Text, UArray etc).

The implementation is essentially the same as for the existing
primops, and shares the memcpy stuff in the code generators.

Defficiencies / future directions: none of these primops (existing
or the new ones) let one take advantage of knowing that ByteArray#s
are word-aligned in memory. Though it is unclear that any of the
code generators would make use of this information unless the size
to copy is also known at compile time.

Signed-off-by: Austin Seipp <austin@well-typed.com>