Implement pxbind fix for combo bit flags with partially unnamed bits

[NiklasNummelin]

Can't run pxbind on my Mac due to #218

Fixes for instance PxFilterFlags::Notify.

Before, the generated Rust code looked like this:
const Notify = Self::Callback.bits;

while the equivalent C++ looks like this:
eNOTIFY = (1<<3) | eCALLBACK,