Added Within function.

Within is useful for finding the bin of a datapoint, for example when making a histogram or otherwise trying to find similar points. This commit adds the function and tests.
Please don't make it inclusive on both sides.  Real data often has a lot of structure in it, and if that includes having a lot of values on the edge of a bin it would produce very unintuitive results.  I think it should be possible to create a partition of the number line with `within` and if you use inclusive bounds on both ends it would be difficult.  I've seen enough of this (from date handling in time series) to know that having asymmetric bounds are much, much easier to work with.