Making f64::from_str case insensitive

[JohnBSmith]

Consider making f64::from_str(src: &str) -> Result<f64, ParseFloatError> case insensitive due to the following reasons:

1. people may see infinity as a constant and write "INF" or "Inf"
2. f64::INFINITY is all upper case
3. f64::NAN is all upper case
4. is_nan is all lower case
5. e in "12e3" is case insensitive, so it seems to be inconsequent
6. "NaN" is camel case, but "inf" is lower case, which seems to be a little bit inconsistent
7. to_ascii_lowercase is slow and will not be of help as "NaN" is mixed case
8. to_ascii_uppercase is slow and will not be of help as "NaN" is mixed case
9. a hypotetical function to_standard_form(src: &str, dest: &mut str) -> Result<(), ParseFloatError> is not possible as str cannot be mutable

So far, only "inf" and "NaN" are allowed.

Cons:

• slows parsers down by a minimal amount
• increases code size by a minimal amount

[clarfonthey]

Note that you can obtain &mut str from a String or &mut [u8]. Nonetheless, this makes sense, and I'll work on it.

[dtolnay]

As discussed in #48886 -- I believe we should keep this the way it is. Thanks anyway!