Rollup merge of rust-lang#77851 - exrook:split-btreemap, r=dtolnay

BTreeMap: refactor Entry out of map.rs into its own file

btree/map.rs is approaching the 3000 line mark, splitting out the entry
code buys about 500 lines of headroom.

I've created this PR because the changes I've made in rust-lang#77438 will push `map.rs` over the 3000 line limit and cause tidy to complain.

I picked `Entry` to factor out because it feels less tightly coupled to the rest of `BTreeMap` than the various iterator implementations.

Related: rust-lang#60302