Optimize beta_reduce in Instantiator.cc
Optimize beta_reduce in Instantiator.cc

Better pass vmap by reference than by copy.

AmeBel/atomspace,rTreutlein/atomspace,rTreutlein/atomspace,rTreutlein/atomspace,AmeBel/atomspace,AmeBel/atomspace,rTreutlein/atomspace,rTreutlein/atomspace,AmeBel/atomspace,AmeBel/atomspace