add type asserts for `MA.operate!` return values

[nsajko]

This helps ensure the MutableArithmetics contract is respected. Specifically, MA.operate! should always return its first argument.

Why this instead of performing a stronger check with ===? Because:

• That would be more verbose
• The === would sooner have a run time cost than a type assertion

[blegat]

Is this improving any benchmark or just some double checks ? In any case, please fix the format error and then it should bz good