The import has been unnecessarily complicated since ParseFloatError::Invalid is not longer used unqualified.
The pfe_* functions do not need to be public any more since the only other use site, from_str_radix for floats, has been removed.
Move private bignum module to core::num, because it is not only used in flt2dec.
Extract private 80-bit soft-float into new core::num module for the same reason.
Stop using stability to hide the implementation details of ParseFloatError and
instead move the error type into the `dec2flt` module. Also move the
implementation blocks of `FromStr for f{32,64}` into `dec2flt` directly.
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]
