SvTRUE(): inline ROK, outline NOK

SvTRUE(): inline ROK, outline NOK

SvTRUE (and its variants) are wrappers around sv_2bool(), which
attempt to test for the common cases without the overhead of a function
call.

This commit changes the definition of common:

SvROK() becomes common: it's very common to test whether a variable
is undef or a ref;
SvNOK becomes uncommon: these days perl prefers IV values over NV values
in SVs whenever possible, so testing the truth value of an NV is less
common.