Calculating gas using usize (if supplied gaslimit fits in usize) (#1518)

* Spliting gasometer out of interpreter

* Choosing right gas calculations implementation based on supplied gas

* Moving verification out of gasometer

* MemGasCost benchmark.

Conflicts:
	ethcore/src/evm/benches/mod.rs

* Some simple benchmarks

* Benchmark for simple loop

* Calculating gas_for_memory only when it's actually needed

* Removing superfluous newline [ci skip]

ethcore/src/json_tests/executive.rs

@@ -208,7 +208,7 @@ fn do_json_test_for(vm_type: &VMType, json_data: &[u8]) -> Vec<String> {
 				&mut tracer,
 				&mut vm_tracer,
 			);
-			let mut evm = vm_factory.create();
+			let mut evm = vm_factory.create(params.gas);
 			let res = evm.exec(params, &mut ex);
 			// a return in finalize will not alter callcreates
 			let callcreates = ex.callcreates.clone();