Greatly condensing the verbose, strangely-ordering `in-radius` definition

The primary purpose of this change is to allow for Tortoise engine's `in-radius` to be small, clear, and fast.  The reason that this PR is necessary is because this "small and fast" version can cause the ordering of `in-radius`'s result to change, which causes the Tortoise tests to yield different results than Headless does.  Initially, you might think that this could be worked around in Tortoise by just having the tests sort the output of `in-radius` (and that did work in some cases), but full model tests using `in-radius` went out of sync, because things like `one-of turtles in-radius 3` would have `one-of` grabbing a different turtle from the same index it was previously taking from.  So, to me, it seems that the place this problem should be solved over in Headless, since I think it's ultimately to the benefit of both Tortoise and Headless to have this code.

Review/merge by @frankduncan?