Improve parsing of numbers in util/str.rs

[KiChjang]

#10608 added support to parse double values from DOMStrings, but the mechanism it uses is messy - we have to keep track of how many digits we've parsed for the units part and the fraction part. We should make use of take_while_ref from the itertools crate.

Code: components/util/str.rs

[SunyDays]

I'd like to work on it.
I understand correctly, that tracking number of digits in unit and fraction parts need add to functions read_fraction and read_exponent for control that value which we parse can be stored in f64?

[KiChjang]

No, that's not the issue here. We want to not keep track of the number of digits parsed in read_fraction or read_numbers; instead, it should simply mutate the iterator, fast-forwarding to the index after the last digit was parsed. take_while_ref from the itertools crate is perfect for this, but it looks like there are some trait bounds that are not satisfied when I first tried to implement it in #10608.

[KiChjang]

Previous attempt at #10974.