strtof(), strtod(): handle NaN and Infinity

There was a TODO in the source for adding NaN and Inf parsing.

My solution is: after sign parsing, if the input string in lowercase starts with 'nan' respective 'inf', set result to corresponding constant from the underlying rust type

We seem to not have negative NaN support in rust. But I am not sure if that would be needed to satisfy the Standard; I don't think so.