simplifications: add s_mp_zero_(digs|buf) and s_mp_copy_digs
Originally I made those as macros. However we have many
other small functions like mp_clamp, mp_exch which are also not implemented
as macros right now.
If we would use c99, I would implement them as private static inline
functions. And mp_exch would be a public static inline function.
But since we are bound to c89, we simply use normal functions.
To achieve optimal performance one should either use link time
optimization or amalgamation.
Execute move.sh - Rename files from bn_* to match the function names.
* git blame <renamed-file> is not affected
* git log --follow <renamed-file> can be used to show log across renames
