this module generally attempts to decode as 64-bit x86 instructions, on
the assumption they are the most likely-desired instructions, falling
back to 32-bit and then 16-bit decoding, in order.

translation from a 64-bit `long_mode::Instruction` to
`generic::Instruction` is close to free, where
`protected_mode::Instruction` and `real_mode::Instruction` may be a
little more costly in time but should still not be too bad.

docs still need much touching up. most docs reference the `long_mode`
structures and enums they're strongly inspired by.

generate_opcode.py has quickly grown into generating much more than just
opcode definitions, and now handles a few duplicate impls across the
different decode modes as well. some of the added impl generation
conflicts with still-existing hand-written impls from yore, so they
needed a bit of removing.

next will be the addition of a generic module for "probably what you
want" disassembly of x86, avoiding the 64-/32-/16-bitness of the
architecture family with an attempt to decode "probably what you wanted"
from a byte sequence. it needs a little more work still, but TODO stubs
added here support that new module.

unfortunately because of the layout of instruction information this
*adds* lines rather than removes them..