feat: EquivBEq and LawfulHashable classes

[TwoFX]

Split from #4583

There are two open questions, opinions appreciated:

• Should this material be part of Init or Std?
• Should the typeclasses be in the Std namespace?

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Mathlib CI can not be attempted yet, as the nightly-testing-2024-07-01 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-mathlib, Mathlib CI should run now. (2024-07-01 12:26:11)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 554e7234330cf06efffe0cb52092e783a27561cc --onto 4d0b7cf66c140dd83f8c8634293047335a385026. (2024-07-02 08:29:48)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8959b2ca87a67cb88ab0828db3672384ad7302fe --onto 4d0b7cf66c140dd83f8c8634293047335a385026. (2024-07-02 12:40:50)

[kim-em]

I think this should be in the root namespace, at least for now.

Let's keep this in Init.

[TwoFX]

Okay, I have moved the classes to Init.