A fast B+ Tree implementation that uses integer keys.
The API is similar to the standard library's BTreeMap with some significant differences:

* Lookups and insertions are 2-4x faster than BTreeMap.
* BTree can optionally be used as a multi-map and hold duplicate keys.
* Keys must be integer types or convertible to integers via the BTreeKey trait.
* The maximum integer value is reserved for internal use and cannot be used by keys.
* Elements in the tree are ordered by the integer value of the key instead of the Ord implementation of the keys.
* Cursors can be used to seek back-and-forth in the tree while inserting or removing elements.
* Iterators only support forward iteration.

Memory safety is all the rage these days. But what does the term even mean? <...> Typically, people use this term to refer to languages that make sure that there are no use-after-free or out-of-bounds memory accesses in the program. This is then often seen as distinct from other notions of safety such as thread safety, which refers to programs that do not have certain kinds of concurrency bugs. However, in this post I will argue that this distinction isn’t all that useful, and that the actual property we want our programs to have is absence of Undefined Behavior.