Make namespace `std` for standard external functions

[Sven Nilsen]

By making binary and unary operators external functions (see #635), it becomes possible to override operators by declaring a function with same name. In order to preserve existing functionality while extending with new functionality, there must be a way to call the standard operators.

For example:

use std as std

// Override the `|<expr>|` operator.
fn norm(a) -> {
    if typeof(a) == "vec4" {
        return std::norm(a)
    } else if typeof(a) == "number" {
        return std::abs(a)
    } else if typeof(a) == "array" {
        return std::len(a)
    }
}

fn main() {
    println(|(1, 0)|)       // Prints `1`
    println(|-1|)           // Prints `1`
    println(|[1, 2, 3]|)    // Prints `3`
}