[Gist - Generalised Search Tree](https://en.wikipedia.org/wiki/GiST)

A Generalised Search Tree is a tree structure similar to B+Trees that has a key
structure that has been abstracted to an interface. This allows it to be used
for various interesting structures like B+Trees, RTrees, BallTrees, and so on,
just by inmplementing the interface for the GstKey. GiSTs were made popular by
Postgres since they allow people to make interesting indices on tables that
efficiently handle things like time intervals, or PostGIS.

This is a work in progress so there are some bits of weirdness.

e.g. It only handles Rects which then have Rects as the keys rather than having
Rects in intermediate nodes which are pointing to Points or whatever.

Also, removal is a bit shonky and empty leaves are left in the tree. But to be
fair, [the original libgist has this issue
too](http://gist.cs.berkeley.edu/libgist-2.0/relnotes.html).

h2. Naming
The name `gist` was taken just four days before I tried uploading this to
Crates.io by a project for uploading Github gists. No one seems to know what the
'i' in GiST stands for anyway.