Replace increase/decrease_allowance functions with approve

[sisuresh]

Update sdk to match SAC changes.

I tried to add the following documentation to the token interface for the approve function, but I was getting a LengthExceedsMax macro error. This should be added to our docs instead.

/// # A note on modifying allowances
///
/// The approve function overwrites the previous value with `amount`, so it's
/// possible for the previous allowance to be spent in an earlier transaction
/// before `amount` is written in a later transaction. The result of this is
/// that `spender` can spend more than intended. This issue can be avoided by
/// first setting the allowance to 0, verifying that the spender didn't spend
/// any portion of the previous allowance, and then setting the allowance to the
/// new desired amount. You can read more about this issue here - https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729.