Pick a better Mach-O parser

[woodruffw]

Right now we vendor the Kaitai Struct project's generated Mach-O parser, which has some disadvantages:

• It only supports "single-arch" Mach-Os, which means we have to do some nasty hacking to parse "fat" Mach-Os
• It has a dependency on Kaitai's ASN.1 parser, for components we don't require. This means vendoring more code than strictly necessary
• It introduces a runtime dependency on kaitaistruct for parsing support

We really only need to parse the symbol table(s), so we could probably get away with a tiny Mach-O parser that only does that.

[woodruffw]

Kaitai has merged support for "fat" Mach-Os: kaitai-io/kaitai_struct_formats#515