message: Hide vec_map::THRESHOLD
  Summary:
  `THRESHOLD` is `VecMap` internals, so:
  * it makes API harder to use
  * not safe (incorrect work with capacity results in panic)
  Previous diffs change API to stop using `THRESHOLD` field, and this diff hides it.
diff: @@ -21,7 +21,7 @@ use indexmap::{Equivalent, IndexMap};
  use std::{fmt, hash::BuildHasher, mem};
  // TODO: benchmark, is this the right threshold
  -pub const THRESHOLD: usize = 12;
  +const THRESHOLD: usize = 12;
  // We define a lot of iterators on top of other iterators
  // so define a helper macro for that
  