Remove math support header and put functions into Nat.hpp as friends

[saxbophone]

Having these in their own separate header has always felt a bit weird. Having them as friend functions makes much more sense.

That way, each arbitrary type has its own log, pow and root function, but they're friends so you don't need to use them with the class name. This provides overloaded behaviour across the different types —i.e. there's an ipow() for Nat, Int and Rat

Note: Consider whether we want to call it ipow() in light of the fact that Rat provides rational arithmetic, so is not integer (i is short for integer).

[saxbophone]

Actually, they don't need to be made friends, they can just exist as free functions within the Nat header