Use macros or inline functions to optimize further (??)

[brodycj]

I started using a macro to optimize cencode.c further in https://github.com/brodybits/libb64-encode and will likely do something for cdecode.c in my own fork. I think recent C compilers can handle inline functions as well, just found this:

• https://stackoverflow.com/questions/31108159/what-is-the-use-of-the-inline-keyword-in-c