Using I: IntoIterator instead of T: IntoIterator in the standard library

[George-Ogden]

Hi

Firstly, apologies if this is the wrong place or if people vehemently disagree with this point. I think that this is important for documentation, as well as consistency.

I searched for the following code snippets in the Rust codebase, and here are the results:

• "T: IntoIterator": 58 files
• "I: IntoIterator": 100 files
• "I: Iterator": 100 files
• "T: Iterator": 100 files

I think for standardisation, I should be used for Iterator traits and IntoIterator traits (then J).
This is already common in the itertools(https://docs.rs/itertools/latest/itertools/) crate, which then uses this as part of the documentation.
In particular, <T, I: IntoIterator<T>> communicates a much clearer intention that <S, T: IntoIterator<S>> as seen here:

impl<S> FromIterator<S> for DiagSymbolList<S> {
    fn from_iter<T: IntoIterator<Item = S>>(iter: T) -> Self {
        iter.into_iter().collect::<Vec<_>>().into()
    }
}

Please let me know your thoughts.