Rollup merge of #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 #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: #60302

clippy_lints/src/utils/paths.rs

@@ -14,7 +14,7 @@ pub const BINARY_HEAP: [&str; 4] = ["alloc", "collections", "binary_heap", "Bina
 pub const BORROW_TRAIT: [&str; 3] = ["core", "borrow", "Borrow"];
 pub const BOX: [&str; 3] = ["alloc", "boxed", "Box"];
 pub const BTREEMAP: [&str; 5] = ["alloc", "collections", "btree", "map", "BTreeMap"];
-pub const BTREEMAP_ENTRY: [&str; 5] = ["alloc", "collections", "btree", "map", "Entry"];
+pub const BTREEMAP_ENTRY: [&str; 6] = ["alloc", "collections", "btree", "map", "entry", "Entry"];
 pub const BTREESET: [&str; 5] = ["alloc", "collections", "btree", "set", "BTreeSet"];
 pub const CLONE_TRAIT: [&str; 3] = ["core", "clone", "Clone"];
 pub const CLONE_TRAIT_METHOD: [&str; 4] = ["core", "clone", "Clone", "clone"];