Auto merge of #98178 - RalfJung:btree-alloc, r=thomcc

btree: avoid forcing the allocator to be a reference

The previous code forces the actual allocator used to be some `&A`. This generalizes the code to allow any `A: Copy`. If people truly want to use a reference, they can use `&A` themselves.

Fixes rust-lang/rust#98176

Author: bors