Switch `GasInt` to `uint64`

[arnetheduck]

The initial choice in https://github.com/status-im/nimbus-eth1/issues/35#issuecomment-391726518 is argued to simplify development by raising a Defect whenever an out-of-bounds computation happens.

While this might have been an appropriate choice in early-stages design, it is not a reliable choice for production code, also because types where GasInt is used are RLP-serialized and RLP doesn't support signed integers.

Since geth uses uint64, this will also ensure that our handling is similar to that of geth:

https://github.com/ethereum/go-ethereum/blob/682ae838b2312a4ec8e5581069039b567e33c7c2/core/types/block.go#L76C2-L76C10

As an alternative to plain uint64, we could also develop a special "checked uint64" in which all operations return an overflow flag that is checked where applicable.

[jangko]

A note: Some of the gas calculation depends on the int signedness, e.g. in evm/interpreter/gas_costs.nim and evm/interpreter/gas_meter.nim and in calculateAndPossiblyRefundGas of nimbus/transaction/call_common.nim are using equations that will result in negative number as intermediate value. We need to rewrite the equations to avoid negative number when fixing this issue.

[arnetheduck]

Something to be careful about: code like https://github.com/status-im/nimbus-eth1/blob/32c51b14a4b574433d1cd0a48b3ebcb37022aa52/nimbus/core/validate.nim#L329 will allow negative values (!)