Change name 'av_top' to 'av_top_index'

 In using the av_top() function created in a recent commit, I found
 myself being confused, and thinking it meant the top element of the
 array, whereas it really means the index of the top element of that
 array.  Since the new name has not appeared in a stable release, it can
 be changed, without remorse, to include 'index' in it.
Inline av_top_index()

This function is just an assert and a macro call.  Avoid the function
call overhead by making it linline.
Add av_tindex() synonym for av_top_index()

The latter is a somewhat less clumsy name.  The old one is provided a a
very clear name; the new one as a somewhat slangy version
Change pods to not refer to av_len()

This name for the function is misleading; don't encourage its use.