[Merge] Enter inline.h

This is a home for static inline functions that cannot go in other
headers because they depend on proto.h or struct definitions.

This allows us to avoid repeating macros with GCC and non-GCC ver-
sions.  It also makes it easier to avoid evaluating macro argu-
ments twice.

I’ve moved just enough things into it to offset the additional lines
added by the comments at the top.  The ‘net code removal’ of this
branch is 4 lines.