3601f9d40b
Constantify more BTreeMap and BTreeSet functions Just because we can: - `BTreeMap::len` - `BTreeMap::is_empty` - `BTreeSet::len` - `BTreeSet::is_empty` Note that I put the `const` under `const_btree_new`, because I don't think their is a need to create another feature flag for that. cc #71835