x86 instruction encoding

I've been working on a [library for encoding x86 instructions](https://github.com/momumi/x86-zig) which should be useful for this project.

Only a few opcodes are implemented at the moment, but the framework is mostly complete. Now it's just a matter of crawling through the Intel datasheets and adding the instructions to a [lookup table](https://github.com/momumi/x86-zig/blob/2400650b5c61ade02e66a423e4986b1c8e283ee0/src/x86/database.zig#L351).

Although it's far from complete I'm mentioning it now to save people from duplicating work.