t.fn should accept unknown in place of arguments

[arcanis]

The trick is that while it should accept unknown, it should only accept unknown (or the actual right type). We don't want to allow passing a string as parameter if we know the type should be a number.

import * as t from 'typanion';

declare const foo: unknown;

const fn = t.fn([t.isNumber()], val => {});
fn(42);
fn(foo);

// @ts-expect-error
fn("foo");