Optimise if (%foo) to be faster than if(keys %foo)

Optimise if (%foo) to be faster than if(keys %foo)

Thread was "[PATCH] Make if (%hash) {} act the same as if (keys %hash) {}"
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2006-11/msg00432.html
but the implementation evolved from the approach described in the subject, to
instead add a new opcode pp_boolkeys, to exactly preserve the existing
behaviour.

Various conflicts with the passage of time resolved, 'register' removed, and a
$VERSION bump.