SV_CONST(name) and PL_sv_consts

SV_CONST(name) and PL_sv_consts

SV_CONST(XXX) returns SV* that contains "XXX" string.
SVs are built on demand and stored in interp's structure
for re-use. All SVs have precomputed hash value.

Creates SVs on demand, we don't want 35 SV created during
compile time or cloned during thread creation.