Rollup merge of #132503 - RalfJung:const-hash-map, r=Amanieu

better test for const HashMap; remove const_hash leftovers

The existing `const_with_hasher` test is kind of silly since the HashMap it constructs can never contain any elements. So this adjusts the test to construct a usable HashMap, which is a bit non-trivial since the default hash builder cannot be built in `const`. `BuildHasherDefault::new()` helps but is unstable (https://github.com/rust-lang/rust/issues/123197), so we also have a test that does not involve that type.

The second commit removes the last remnants of https://github.com/rust-lang/rust/issues/104061, since they aren't actually useful -- without const traits, you can't do any hashing in `const`.

Cc ``@rust-lang/libs-api`` ``@rust-lang/wg-const-eval``
Closes #104061
Related to https://github.com/rust-lang/rust/issues/102575

library/std/src/lib.rs

@@ -328,6 +328,7 @@
 // Library features (core):
 // tidy-alphabetical-start
 #![feature(array_chunks)]
+#![feature(build_hasher_default_const_new)]
 #![feature(c_str_module)]
 #![feature(char_internals)]
 #![feature(clone_to_uninit)]
@@ -415,7 +416,6 @@
 // Only for const-ness:
 // tidy-alphabetical-start
 #![feature(const_collections_with_hasher)]
-#![feature(const_hash)]
 #![feature(thread_local_internals)]
 // tidy-alphabetical-end
 //