Introduce new constraint for register pair

[kito-cheng]

Must start with an even-numbered register.

NOTE: This is NOT implemented on either on LLVM or GCC yet, this PR will hold until at least one compiler has implemented.

Fix #37

[cmuellner]

There is a decision between two options:

• an even-numbered register
• a register pair that is identified by the even, lower-numbered register

The difference is, that the second implies the amount of registers in the constraint, while the other one does not (or implies 1 if not stated otherwise).

#37 requests the first option. This PR introduces the second. Is there a reason to go for the second option besides personal preference (I admit that I prefer the second option for this reason)?

And would clobbering a register with the constraint "G" imply that a register pair is clobbered (is this easy to implement in GCC/LLVM)?

[lenary]

I see my proposal in #92 conflicts with this, as it proposes a different name (Pr vs R). I chose Pr because I felt it was extensible to other kinds of paired registers in the future, potentially.

[kito-cheng]

@lenary Yeah, I agree this should be designed for more extensibility, and your proposal is better than this one :) so let me drop this proposal to prevent misleading that we have two competing proposals.