395252c6d9 Fix FeeRate::checked_add/sub (Tobin C. Harding)
Pull request description:
Currently the `checked_add` and `checked_sub` functions use a `u64` for the right hand side. This leaks the inner unit because the RHS value is implicitly in the same unit.
Make the functions have `FeeRate` on the RHS.
ACKs for top commit:
apoelstra:
ACK 395252c6d95fdd581ce5e8ad5d7978515aafea23; successfully ran local tests; will one-ACK merge as this is units-only
Tree-SHA512: 1f0893f75c47f720ac741ace0274171ed24efcb6d2724d0fed941899d43b213e165b97aa050d6e40eea78c527af45d090a81b9d6cbd95835ef7105585786fca6