add BITS associated constant to PrimInt

[umanwizard]

[cuviper]

Unfortunately, this is a breaking change for external implementors, especially since this trait isn't sealed.

It's tempting to add a default to 8 * mem::size_of::<Self>(), but nothing says that the entire value has to be numeric bits either. It could be a wrapper with some other extraneous metadata, for example.

#247 suggests a new trait, which would avoid a breaking change.