Add newtype for registers

[philipc]

• Add a struct Register(u16). u16 is chosen because the ARM ABI specifies values larger than 256. Replace all register usage with the newtype. Previously we used u8 for CFI and u64 for expressions.

• Add constants for X86/X86-64 and some ARM registers. Add register_name methods. These are defined within empty structs rather than modules. This gives us the option in future of implementing a trait for these structs (eg register_name may make sense as a trait method, and maybe things like a fn sp() -> Register, although that may be better in the unwinder).

• Update dwarfdump to use the register names when dumping .eh_frame (but not expressions because they usually have register specific opcodes rather than register numbers).

Fixes #200

[fitzgen]

Nice!