The motivation is to allow functions that use FloatExtractor to be
constexpr. Type punning through a union will never work in constexpr.
In practice, bit_cast<>()ing bit fields also does not yet work in clang,
but that's just a bug and it will work eventually (and it does already
work in gcc): https://github.com/llvm/llvm-project/issues/54018
No behavior change.
Currently, the floating point to string conversion is implemented
several times across the codebase. This commit provides a pretty
low-level function to unify all of such conversions. It converts the
given double to a fixed point decimal satisfying a few correctness
criteria.
