Add experimental XED decoder bindings

[aengelke]

This add bindings to the Intel XED decoder. Note that:

1. Some instructions (especially instructions accessing a half vector register) are decoded differently
2. Pass-through is not implemented
3. Many cases are (probably) not handled yet
4. There might by bugs as this has not been tested in depth yet.

On the other side, with a bit more work this would give us a well-tested decoder, which covers almost all instructions.

/cc @weidendo

[weidendo]

Cool.

I expected that filling out DBrew decoder structs would be more difficult to do.
To keep this small, we could align the DBrew decoder structs more to what XED is doing.

As API, it should be enough to have dbrew_setdecoder(void* decode_dbb).
How far are we from passing the tests?

Regarding marking an instruction as pass-through: I really would like to get rid of that,
and have proper support in the rewriter (I have some pending changes in a local branch).