Revert "Update P6int to properly read sized ints."

This reverts commit 0b7de0c193e25059b3e79d2d67d0f9e01d61ffbf.
Better sized int handling in P6int.
Better getting/setting of natively typed attributes.

This brings natively typed attributes more in line with the approach used in
NQP-JVM. Instead of get_attribute and bind_attribute dealing directly with
pointers to the data, we use a NativeValue struct to pass values. This way,
the class can use the attribute's REPR to get and set values.