Rollup merge of #52402 - crepererum:build_hasher_eq, r=sfackler
Rollup merge of #52402 - crepererum:build_hasher_eq, r=sfackler

impl PartialEq+Eq for BuildHasherDefault

`BuildHasherDefault`is only one way of implementing `BuildHasher`. Clearly, every `BuildHasherDefault` for the same type `H` is identical, since it just uses `Default<H>` to construct `H`. In general, this is not true for every `BuildHasher`, so I think it is helpful to implement `PartialEq` and `Eq`.

GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust