Proposal: Add FromIterator impl for HashMap<K, Vec<V>>

Summary

I'm proposing a small quality of life improvement, that I was surprised to find wasn't already a part of the language.
I want to introduce a new FromIterator<(K, V)> implementation for HashMap<K, Vec<V>>.

I'm happy to create a PR for this if we want to move forward.

Note: I tried asking this on internals, without any responses, so am hoping to get more traction here.

Motivation

It's relatively common to want to group a list of values over some specific key. The current method of doing this either requires a third party crate or manually constructing the HashMap in a for loop, making use of the Entry API.

This is common enough that I was surprised not to see an ergonomic approach using collect. This RFC proposes adding an impl to make this possible.

Example

As an example, imagine you had a list of books, and wanted to group the books by the author that wrote them.

Current Implementation

let mut books_by_author: HashMap<&str, Vec<&Book>> = HashMap::new();  
  
for book in &books {  
    books_by_author.entry(book.author).or_default().push(book);  
}

With This Proposal

let books_by_author: HashMap<&str, Vec<&Book>> =
    books.iter().map(|book| (book.author, book)).collect();

Future Considerations

• Instead of adding this implementation for just Vec<V>, we could consider adding it for all collections C: Default + Extend<V>, extending this functionality to all collections, such as a HashSet.

Prior Art

• multimap
• itertools::into_group_map

Start with the proposal's FromIterator<HashMap<K, Vec>> examples and compare the existing HashMap Entry API with the linked multimap and itertools prior art. Determine whether the narrow implementation or the broader Default + Extend design fits the RFC process; done means a resolved RFC direction rather than an implementation patch.