a1ce2d1ac8 Use _u32 in FeeRate constructor instead of _unchecked (Tobin C. Harding)
Pull request description:
When constructing an `Amount` it was observed recently that a `u32` is often big enough. For the same reason we can use a `u32` to construct a fee rate instead of overflowing.
Use `_u32`in the constructor and remove the panic.
ACKs for top commit:
apoelstra:
ACK a1ce2d1ac8d646b63532ed9ac459ffea39ec8c20; successfully ran local tests
Tree-SHA512: 339974c954ad613b0be684f7b2fa1402f59fe401969f3785a702ffbb14ac913ecf4c8228240d068ba4b482e38263590154167a071d823ccc9b4d0691036ca484