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.

this is also checked by a new fuzz target

also adjust changelog for a 1.2.1 version again, no new interfaces to go
with these bugfixes.

registers `al`, `cl`, `dl`, and `bl` could have two different
representations - with `rex.w` and without. these two forms of `RegSpec`
would not compare equal, nor has the same, so for code relying on
`RegSpec` to faithfully represent a 1-1 mapping to x86 registers, these
synonyms would introduce bugs in register analysis.

for example, in `yaxpeax-core`, this would result in instructions
writing to `rex.w al` not being visible as definitions for a future
read of `!rex.w al`.

fix this in `x86_64` code, add new test cases about the confusion,
adjust register names to make this situation more clearly a bug, and
introduce two new fuzz targets that would have helped spot this error.