General purpose registers should be distinguished from floating-point ones

[kulp]

At present, lightning-sys uses a general Reg type to represent a register of type R, V, or F. Although R (caller-save) and V (callee-save) registers can be used interchangeably, F (floating-point) registers are in a distinct class. lightning-sys should use different types (perhaps Gpr and Flt instead of Reg) to represent the distinction between integer and floating-point registers. Doing so will improve type-safety, preventing certain classes of errors.

[kulp]

I have previously done some experiments in this area, and I plan on working on this myself sometime after the completion of #43.