Complete `Num` instance for `Term`

[MaximilianAlgehed]

It should be straight-forward to implement abs_ and signum_ and worth it to avoid a partial Num instance.

[MaximilianAlgehed]

It turns out doing abs in a satisfactory manner is harder than you'd thing. Signum is easy though.

[MaximilianAlgehed]

C.f. this comment in #40:

-- The reason there is no implementation of abs here is that you can't easily deal with abs
-- without specifications becoming very large. Consider the following example:
-- > constrained $ \ x -> [1000 <. abs_ x, abs_ x <. 1050]
-- The natural `Specification` here would be something like `(-1050, -1000) || (1000, 1050)`
-- - the disjoint union of two open, non-overlapping, intervals. However, this doesn't work
-- because number type-specs only support a single interval. You could fudge it in all sorts of ways
-- by using `chooseSpec` or by using the can't set (which would blow up to be 2000 elements large in this
-- case). In short, there is no _satisfactory_ solution here.