Make string conversion functions mappings

[Timmmm]

Currently we have these string conversion functions/mappings (ignoring hex_bits_signed):

Name	Base	Type
hex_bits	16	bits(N) <-> str
bits_str	16 (or 2 if N is not a multiple of 4)	bits(N) -> str
dec_str	10	int -> str
hex_str	16	int -> str

This is a bit inconsistent. It would be nice if everything was a mapping, and maybe the naming was a bit more consistent:

Name	Base	Type
hex_bits	16	bits(N) <-> str
str_bits?	16 (or 2 if N is not a multiple of 4)	bits(N) <-> str
dec_int	10	int <-> str
hex_int	16	int <-> str

I dunno if those are the best names. Anyway making dec_str and hex_str a mapping would be useful in quite a lot of the RISC-V assembly mappings.