Merge the direct structure lookup for B::IV:: and B::NV:: into 1 routine.

Merge the direct structure lookup for B::IV:: and B::NV:: into 1 routine.

As for OPs and GPs, encode the type and offset in the XS alias integer.
Use different names for the type macros, to give them different constant
values, to allow the 3 accessor routines to have no gaps in the values used
for the set of types that they deal with. Otherwise, at least 1 switch
statement would compile to a jump table with "holes".

On this platform, this refactoring reduces the object code size by 4.5K.