The intent is to allow decimal-to-float parsing to use Fp in its fast path.
That code is added in a later commit.

This is necessary for decimal-to-float code (in a later commit) to handle
inputs such as 4.9406564584124654e-324 (the smallest subnormal f64).
According to the benchmarks for flt2dec::dragon, this does not
affect performance measurably. It probably uses slightly more stack
space though.

- Exposing digits and individual bits
- Counting the number of bits
- Add small (digit-sized) values
- Multiplication by power of 5
- Division with remainder

All are necessary for decimal to floating point conversions.
All but the most trivial ones come with tests.

fast decimal-to-float conversions.

This commit primarily adds implementations of the algorithms from William
Clinger's paper "How to Read Floating Point Numbers Accurately". It also
includes a lot of infrastructure necessary for those algorithms, and some
unit tests.

Since these algorithms reject a few (extreme) inputs that were previously
accepted, this could be seen as a [breaking-change]

Running these tests takes hours, so they are not run by @bors.