and your done write it once and it works every time no needless complexity.
At the beginning of every file put #include "z80 lib common.h" and your done it all works.
I'm honestly surprised it doesn't already exist.
and you don't have to worry too much about making really efficient code cause any decent optimizing compiler will rewrite your code in to something more efficient then you could ever possibly come up with anyway so all that matters is the algorithm you use for the most part.

someone make it weeks project I would do it but I don't have the time with other stuff.  

I am not understanding. Maybe if you give a example of usage?
The few z80 compilers don't produce very good code and that I know there is no optimizer for z80.
If you mean an API consisting of macros for z80, it does exist.

And I was starting to do a z80 lib with common routines like multiplication and division, so if you have some ideas I would like to know.