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]