Better register handling

[EllipticEllipsis]

Need a better way to consider register behaviour, e.g. being clobbered after function calls, and in particular, both avoid string comparisons and deal with different ABIs sensibly.

[AngheloAlf]

Related conversation: ethteck/splat#148 (comment)

[EllipticEllipsis]

Idea: make a class for registers that can have appropriate attributes. Convert instructions to use objects from this class instead of raw numbers.
Pros:

• More OO (which some people think is good).
• Can customise individual registers a bit.
• Can add as many attributes as desired without needing to change much beyond initialisation.
• Possibly only need to know the ABI when creating the register information in the first place?

Cons:

• Might be slower.
• Have to define every register separately.

[EllipticEllipsis]

Idea: ABI-indexed dict of sets for each category.

Pros:

• Can continue to use raw numbers/enums.

Cons:

• Have to always carry the ABI around.

[AngheloAlf]

Cons:

* Have to always carry the ABI around.

The current ABI is stored on InstructionConfig.GPR_ABI_NAMES and InstructionConfig.FPR_ABI_NAMES, which are accessible everywhere, so that shouldn't be a problem