Support for custom hasher for std groupping

[a14e]

Overview

I want to suggest an improvement for the library -- the ability to use custom hashes (specifically interested in ahash).

Why?

The standard library uses a sip hash, which is protected against various types of attacks on hash tables, but it is known to be quite slow. Alternative hashes can be 10+ times faster (for example, there is a benchmark example in ahash)

What difficulties might arise?

It's unclear how to implement this

1. Option 1. Keep the current implementation, but add a third argument with a default value, for example, the methods would look like this:

   #[cfg(feature = "use_std")]
       fn into_group_map<K, V, R = RandomState>(self) -> HashMap<K, Vec<V>, R>
       where
           Self: Iterator<Item = (K, V)> + Sized,
           K: Hash + Eq,
           R: BuildHasher + Default
       {
           group_map::into_group_map(self)
       }

   but the current Rust doesn’t know how to do this and writes something like

   error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
       --> src/lib.rs:3076:29
        |
   3076 |     fn into_group_map<K, V, R = RandomState>(self) -> HashMap<K, Vec<V>, R>
        |                             ^^^^^^^^^^^^^^^
        |
        = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
        = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
        = note: `#[deny(invalid_type_param_default)]` on by default

2. Option 2, we can try replacing all imports with a flag from std::collections::HashMap to ahash::HashMap, but this breaks backward compatibility

Maybe there are some other options?

[a14e]

This issue seems to be a duplicate of #322.

[a14e]

Other possible options:

1. Generalize the current implementation and create duplicates of methods with custom hashers (for example, HashMap has a constructor new and with_hasher).
2. Create some kind of middleware, and call, for instance, vec![1, 2, 3].iter().grouping::<ahash::HashMap>, and implement all grouping options on top of this middleware. What do you think?

[Philippe-Cholet]

As a maintainer or not, I certainly would like to customize the hasher too.
Related to #462 which contains comments written by phimuemue that I agree with and that could interest you.

Maybe we should create a label for this kind of issues?