This makes both of the following return Err:
".".parse::<f32>()
".".parse::<f64>()
This is a [breaking-change], which the libs team have classified as a
bug fix.
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]