Wrote an alternate BTreeMap with const Layout

via

The Rust Programming Language

Due to the need for special cursor api, (and cursor feature in std is not stable yet), I've spent a month writing an alternate version of BTreeMap. The intended scenarios are for integer keys with a large and long-lived dataset. The result is satisfying. doc: repo: I have done a little investigation of std implementation before I started: The std impl is pure btree (not b+tree) without horizontal links. Each key store only once at either leaf and inter nodes. The std impl is optimised for point