these all existed since forever but the library did not distinguish them
and did not provide prefix information for users to tell which had been
decoded.

along the way, fix an error: maskmov is memory read-write.
additionally, operand information about {push,pop}a{,d}.

this is a squash of a few months' hacking, including but not limited to
what eventually got extracted into
https://git.iximeow.net/asmlinator/about/

the path here is generally not historically interesting, and the vast
majority of this diff is very particular static data tables
(BehaviorDigests and implicit operand lists)

`src/long_mode/behavior.rs` will more or less be directly adapted into
versions for x86-32 and x86-16, similar to the instruction decoders.