add intersects method to Interval
fix BigDecimal bug with negative integer exponents
use some covariance in kleene example
Add abs, min, max, sqrt, and nroot to Interval.

This gets us closer to having a fully-operational interval
implementation. There are still some things it might be nice
to have (e.g. trig stuff) but we're getting there.

Also, it would be nice to figure out a good way to talk
about possible equality, comparison, etc when compared to
certain equality, comparison, etc. (For example, only
points and possibly empty intervals can be definitely
equal, but intervals that intersect can be possibly equal.)
Small updates to the GUIDE.