Hash lookup of constant strings optimization:

Hash lookup of constant strings optimization:
Introduce SvREADONLY && SvFAKE to flag an SV which has SvPVX pointing
to string table (as per sharepvn). Add newSV_pvn_share to create such
a thing. Make hv.c compare addresses of strings and skip string compare
if equal. Make method_named and helem ops use these shared-string SVs
when arg is constant. Make keys op return shared-string SVs (less clearly
a win).

p4raw-id: //depot/perl@7016