Add simple numerical methods for root finding and differentiation

[sharkdp]

This PR adds three functions:

• fn root_bisect<A: Dim, B: Dim>(f: Fn[(A) -> B], x1: A, x2: A, x_tolerance: A, y_tolerance: B) -> A
• fn root_newton<A: Dim, B: Dim>(f: Fn[(A) -> B], f_prime: Fn[(A) -> B/A], x0: A, y_tolerance: B) -> A
• fn diff<X: Dim, Y: Dim>(f: Fn[(X) -> Y], x: X) -> Y / X

for simple numerical tasks