Instruction debug fmt improvements

[Dentosal]

Currently the reason.instruction sub-field of PanicInstruction is just a raw integer representation of the instruction bytes. This PR changes the Debug implementation to decode the instruction when possible. It will also show the bytes in hexadecimal format, which is more useful for debugging as it can be used to spot the instruction in a hexdump, and it shows the opcode byte separately.

This PR also changes the debug format of register (non-immediate) operands to be hexadecimal, as it is the established convention (e.g. the specification lists registers using hex values, and do does most of the test code).

Before

reason: PanicInstruction {
    reason: OutOfGas,
    instruction: 1920991264,
},

reason: PanicInstruction {
    reason: ErrorFlag,
    instruction: 0,
},

After

reason: PanicInstruction {
    reason: OutOfGas,
    instruction: MOVI { dst: 0x20, val: 32 } (bytes 72 80 00 20),
}

reason: PanicInstruction {
    reason: ErrorFlag,
    instruction: Unknown (bytes 00 00 00 00),
},

[MitchTurner]

LGTM. I just left a couple nits.
I hadn't thought of just adding it to the Debug. This seems like a good way to get what we wanted without touching much code.

[MitchTurner]

Nit: I think I'd prefer bytes: .

[Dentosal]

Ok, changed.

[MitchTurner]

Nit: Maybe something like Unknown Instruction or Unknown Operation might be slightly more clear when the caller doesn't know what to expect in this field. I don't feel strongly about it though.

[Dentosal]

It already says instruction: Unknown, so I think it's good enough

[xgreenx]

I like both variants=D