astr: use alien instead find/find_substr to speed up cursor movement.

This gives a good order of magnitude speed up on moderate to large
size files, making zmacs usable again :)
* lib/zile/astr.lua (memchr): New function to wrap
`alien.default.memchr`.
(memrchr): New function to simulate GNU memrchr over an alien
array.
(AStr:find, AStr:rfind): Use them.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>