@typeOf() should take multiple parameters and do peer type resolution

[thejoshwolfe]

the type of:

const foo = switch (bar()) {
    A => a(),
    B => b(),
    C => c(),
}

is some type that each of a(), b(), and c() either are or can implicitly convert to. Determining this type is called peer type resolution. That's already a solved problem.

The @typeOf() builtin function should take multiple parameters and do peer type resolution on them to expose this concept directly to users.

This is useful in this context:

fn max(a: var, b: var) -> @typeOf(a, b) {
    if (a > b) a else b
}