Use stack arrays in Elem::operator=,key
Use stack arrays in Elem::operator=,key

This ought to shave 4 or 5 seconds off of the results @friedmud shared
in #1821

dschwen/libmesh,giorgiobornia/libmesh,pbauman/libmesh,capitalaslash/libmesh,hrittich/libmesh,BalticPinguin/libmesh,giorgiobornia/libmesh,pbauman/libmesh,90jrong/libmesh,giorgiobornia/libmesh,hrittich/libmesh,pbauman/libmesh,90jrong/libmesh,BalticPinguin/libmesh,dschwen/libmesh,giorgiobornia/libmesh,pbauman/libmesh,hrittich/libmesh,pbauman/libmesh,hrittich/libmesh,capitalaslash/libmesh,90jrong/libmesh,pbauman/libmesh,libMesh/libmesh,libMesh/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,roystgnr/libmesh,dschwen/libmesh,BalticPinguin/libmesh,BalticPinguin/libmesh,dschwen/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,roystgnr/libmesh,hrittich/libmesh,90jrong/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,roystgnr/libmesh,BalticPinguin/libmesh,dschwen/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,roystgnr/libmesh,90jrong/libmesh,libMesh/libmesh,giorgiobornia/libmesh,hrittich/libmesh,hrittich/libmesh,hrittich/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,roystgnr/libmesh,libMesh/libmesh,libMesh/libmesh,giorgiobornia/libmesh,libMesh/libmesh,roystgnr/libmesh,dschwen/libmesh,90jrong/libmesh,90jrong/libmesh,libMesh/libmesh,hrittich/libmesh,BalticPinguin/libmesh,capitalaslash/libmesh,dschwen/libmesh,roystgnr/libmesh,giorgiobornia/libmesh,90jrong/libmesh,pbauman/libmesh,libMesh/libmesh,capitalaslash/libmesh,jwpeterson/libmesh,pbauman/libmesh,roystgnr/libmesh,capitalaslash/libmesh,dschwen/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,pbauman/libmesh,90jrong/libmesh