EvmStack native endian

[Scooletz]

This PR changes the endianness of the EvmStack to be aligned with the platform it runs on. It means that no endianness conversions should be done when retrieving storing UInt256. The penalty though is related to pushing/poping bytes that now require reversing. According to the profiling this should happen much less frequent than regular operations.

Beside the endiannes, this PR add one method that allows to peek a ref UInt256 to the top of the stack. This allows for any Instruction that has push/pop behavior to get a ref to the last parameter and directly set the value on the stack, without performing calculations for pop and push. Instead it will calculated the offset once for getting the ref and use it for pop.

Changes

• EvmStack uses now native platform endianess
• EvmStack allows peeking a ref to the top

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[Scooletz]

Benchmarks showed no more than 2% gain, which was much less than the standard deviation for the MGas/s. Closing in its current form.