Misc. Cleanups

- Enforce constraint on use of += and -= when an `STAmount` is on the left-hand side and a `std::uint64_t` is on the right hand side: the left-hand side should be a native `STAmount`.
- Prevent implicit conversion of `std::uint64_t` to `uint160` and `uint256`. 
- Don't allow direct conversion of a uint256 into a uint128 - use a function to make the intent clear.