Small improvements to new regidx newtypes

[Timmmm]

Now that #617 is merged I think there are a few little things we could do to improve the code:

1. Add operators for regidx and remove regidx_offset.
2. Add overload bits_of = {...} for regidx_bits, physaddr_bits, etc.
3. Maybe use the same case for newtype regidx : Regidx, i.e. newtype regidx : regidx. I think it's a bit less to think about. physaddr/virtaddr does that.

[Timmmm]

3 was discussed in the meeting. @Alasdair prefers to change physaddr/virtaddr case to be Physaddr etc. so we'll do that instead.

[nwf]

Removing regidx_offset might make certain future affordances more of a nuisance, requiring sign-extension at call-sites. That might be fine, but just to say.