Refactor get_attr_{int,num} (which comes in two variants).

They shared some code that takes a pointer and a number of bits and
dereferences the pointer as an appropriately sized int/num, so refactor those
bits to shared functions.
Kill a piece of dead code.
Refactor how get_attribute_ref works.

Now, get_attribute_ref works more like at_pos_ref, by way of
get_attribute_stable (a new function), so that decoding pointers to embedded
type is delegated to the relevant type, rather than done in nqp.ops. This
eliminates the need for get_attribute_ref to return the number of bits in the
type to the caller via an inout parameter.

Also, this commit temporarily breaks sized int support in CStruct.
Update P6int to properly read sized ints.

This restores sized ints in Zavolaj.

There's one caveat to the current implementation: the code won't compile on
platforms without a 64 bit int type (Parrot_Int8). However, #parrot assures me
that if this type doesn't exist, it's a pretty weird platform.