BuildHasherDefault: remove

[newAM]

This type existed because core::hash::BuildHasherDefault did not have a const constructor.

As of 1.85 core::hash::BuildHasherDefault has a const constructor.

[eldruin]

Thank you!
Is there a workaround for anyone using Rust < 1.85.0 ?
Ferrocene is on 1.83.0, IIRC.

[newAM]

Ferrocene is indeed on 1.83.0.

Ferrocene is about 3 months behind the latest stable, and I don't see heapless 1.0 happening within that time (though I expect hash32 would happen within that time).

There are some options here, I'm happy to implement any of these.

1. Proceed with this PR, and have a gap in ferrocene support for const constructors
2. Keep the type for 1.0, but mark it as deprecated. I need to check if deprecation warnings can be suppressed by users of the crate, otherwise it's similar enough to removing it.
3. Keep the type for 1.0, document core::hash::BuildHasherDefault is preferred on newer toolchains. This does mean maintaining the type for the foreseeable future, but it is a small amount of code.

What do you think?

[eldruin]

I see, thanks.
Let's go with option 1. We can add it back later on if somebody really needs it.