perf: emit instruction's opcodes, not instructions

Lower build times drastically.

Previously, the rust struct `LabelledInstruction` was
emitted. Now, the instruction's raw opcode as well as potential
arguments are emitted as `u64`s. They are then re-interpreted into
`LabelledInstruction`s.

Presumably this works because the rust compiler does not try to analyze
the `u64`s for inlining possibilities, among other things.