0f62c9a582 units: Make minor improvements to MathOp (Tobin C. Harding)
Pull request description:
Follow up from #4312, improve the `MathOp` type by:
- Do not provide public constructor
- Add cast protection
ACKs for top commit:
apoelstra:
ACK 0f62c9a5822a813be5c6f6b19d160458243f174a; successfully ran local tests
Kixunil:
ACK 0f62c9a582
Tree-SHA512: dbb06c5afd8df5364a2aec12b7c8632620a1e8f3955b83e91a9b4f5c2e0daaa1ecdb050d8e395e95bf018d718847cddbf3338bd89f70cbb0382bf5e080d5cf21