py: Combine duplicated code that converts members from a lookup.
Despite initial guess, this code factoring does not hamper performance. In fact it seems to improve speed by a little: running pystone(1.2) on pyboard (which gives a very stable result) this patch takes pystones from 1729.51 up to 1742.16. Also, pystones on x64 increase by around the same proportion (but it's much noisier). Taking a look at the generated machine code, stack usage with this patch is unchanged, and call is tail-optimised with all arguments in registers. Code size decreases by about 50 bytes on Thumb2 archs.