inline.h: Change fcn name prefix from S_ to Perl_

inline.h: Change fcn name prefix from S_ to Perl_

This is being done only for those functions that don't have a guard
preventing them from being seen outside of the Perl core.

Talking to Tony Cook, we agreed that this was a good idea for two
reasons:

1) The 'Perl_' prefix does not pollute XS caller's name space.  The 'S_'
   one could be argued that it doesn't much either, but it does more so
   than 'Perl_', and the next reason is the clincher:
2) It allows us to change our minds about whether a function should be
   static inline or not, without affecting callers who use the Perl_
   form, which they would be accustomed to anyway if they're using the
   full name form.