replace `safeApprove` with `forceApprove`

[ItsNickBarry]

The existing safeApprove function is redundant at best (for reasons explained in the @dev natspec tag) and confusing at worst. This branch removes the zero-balance check and renames the function to forceApprove to prevent silent failure for users who update Solidstate. The name may be reverted to safeApprove in a future update, for consistency with the other SafeERC20 functions.

The SafeERC20 library does not have any tests, so thorough review is required.