Stabilize const BTree{Map,Set}::new

Since `len` and `is_empty` are not const stable yet, this also
creates a new feature for them since they previously used the same
`const_btree_new` feature.

compiler/rustc_hir/src/lib.rs

@@ -4,7 +4,7 @@
 
 #![feature(associated_type_defaults)]
 #![feature(closure_track_caller)]
-#![feature(const_btree_new)]
+#![feature(const_btree_len)]
 #![cfg_attr(bootstrap, feature(let_else))]
 #![feature(once_cell)]
 #![feature(min_specialization)]