Change is required for future dependency updates.

The feature will be removed in a future release of uom. Functionality
previously exposed by the feature is now enabled by default.

Change required the MSRV update as well as refactoring `from_base`,
`to_base`, and multiple tests. The `from_base` and `to_base` changes
provide better floating point precision while still maintaing zero-cost
guarantees. The test changes better handle floating point precision
issues as well as the wider range of values generated by `quickcheck`
1.0's `Arbitrary` implementation.