Add math/lcm (least common multiple) function

[oittaa]

It would be nice to have an official least common multiple (LCM) implementation in the math library. I saw that the crypto module has implemented it, but maybe a better place would be in the math module?

It could be something like the following and being functionally similar with other popular languages. If a zero is passed, many popular programming languages return zero in those cases.

const std = @import("std");

pub fn lcm(a: anytype, b: anytype) @TypeOf(a, b) {
    // Behavior from C++ and Python
    // If an argument is zero, then the returned value is 0.
    if (a == 0 or b == 0) return 0;
    return @abs(b) * (@abs(a) / std.math.gcd(@abs(a), @abs(b)));
}

test lcm {
    const expectEqual = std.testing.expectEqual;

    try expectEqual(lcm(0, 0), 0);
    try expectEqual(lcm(1, 0), 0);
    try expectEqual(lcm(-1, 0), 0);
    try expectEqual(lcm(0, 1), 0);
    try expectEqual(lcm(0, 1), 0);
    try expectEqual(lcm(7, 1), 7);
    try expectEqual(lcm(7, -1), 7);
    try expectEqual(lcm(-23, 15), 345);
    try expectEqual(lcm(120, 84), 840);
    try expectEqual(lcm(84, -120), 840);
}

I would've liked to put @abs just around the whole result value, but gcd didn't like negative values:

/opt/homebrew/Cellar/zig/0.13.0/lib/zig/std/math/gcd.zig:14:29: note: called from here
            std.debug.assert(b >= 0);