core, core/vm, tests: changed the initialisation behaviour of the EVM

The EVM was previously initialised and created for every CALL, CALLCODE,
DELEGATECALL and CREATE. This PR changes this behaviour so that the same
EVM can be used through the session and beyond as long as the
Environment sticks around.

core/vm/instructions.go

@@ -597,7 +597,6 @@ func opDelegateCall(instr instruction, pc *uint64, env Environment, contract *Co
 	toAddr := common.BigToAddress(to)
 	args := memory.Get(inOffset.Int64(), inSize.Int64())
 	ret, err := env.DelegateCall(contract, toAddr, args, gas, contract.Price)
-
 	if err != nil {
 		stack.push(new(big.Int))
 	} else {