Unify types with Geth

[LukaszRozmej]

(Transaction) Nonce: UInt256->ulong
(Transaction) GasLimit long->ulong
(Transaction) SpentGas long->ulong

(Receipt) GasUsed long->ulong
(Receipt) GasUsedTotal long->ulong

(BlockHeader) GasLimit long->ulong
(BlockHeader) GasUsed long->ulong

Could use discussion:
(BlockHeader) Number long->ulong/UInt256: Geth uses big.Int so UInt256, but IMO there is no point and ulong would suffice. It will also speed up any lookups in hashmaps ect and save some memory space.
(Receipt) BlockNumber - as above

[LukaszRozmej]

This issue touches a lot of places in the codebase and needs attention to detail and avoiding type conversions where possible, but is generally simple, so marked as good first issue