Add control flow operators for iterated functions

Add the following control flow operators for iterated functions:

# iterate : ∀a. ℕ → (a → a) → (a → a)
def iterate(n, f, x):
    for _ in range(n):
        x = f(x)
    return x

# orbit : ∀a. ℕ → (a → a) → (a → [a])
def orbit(n, f, x):
    xs = [x]
    for _ in range(n):
        x = f(x)
        xs.append(x)
    return xs

iterate and fori_loop are mutually definable, but the former has a simpler signature and semantics that is more common in my experience, and is arguably more “basic” or “fundamental”.

[jakevdp]

Sure, but adding new APIs does not come without maintenance costs. It’s true that while_loop and fori_loop can be implemented in terms of scan, but their implementations are far more involved than a single line.

As this is the first time I’m aware of receiving such a request, I would lean toward not including them in the package API, but I’m happy to change my mind if there are compelling reasons to do so.

[jakevdp]

I think that could be an improvement – I’d want to hear opinions from other folks on the team