Support Gas costs and balance >= 2^31 on 32-bit

[mratsim]

#40 is introduce native integer for gas prices and balances via int64.

Internally it uses toInt proc from stint implemented as follow:

func toInt*(num: Stint or StUint): int {.inline.}=
  # Returns as int. Result is modulo 2^(sizeof(int)
  num.data.least_significant_word.int

On 32-bit, int32 would be returned instead of int64.

TODO: introduce a toInt64 conversion in Stint

Optionally, toInt64 might be wrapped in Nimbus in a toGasInt proc that will add logging and range check for example to make sure a gas value on the stack (uint256) is not over 2^63 - 1 before conversion.