math: remove usingnamespace for mixins

[slimsag]

suggested to me by mlugg', we could do either:

pub const Vec2 = struct {
    pub fn init(xs: u32, ys: u32) Vec2 {
        return .{ .v = .{ xs, ys } };
    }
    const Shared = SharedImpl(@This());
    pub const add = Shared.add;
    pub const sub = Shared.sub;
};

pub const Vec3 = struct {
    pub fn init(xs: u32, ys: u32, zs: u32) Vec2 {
        return .{ .v = .{ xs, ys, zs } };
    }
    const Shared = SharedImpl(@This());
    pub const add = Shared.add;
    pub const sub = Shared.sub;
};

fn SharedImpl(comptime T: type) type {
    return struct {
        fn add(a: *const T, b: *const T) T {
            return .{ .v = a.v + b.v };
        }
        fn sub(a: *const T, b: *const T) T {
            return .{ .v = a.v - b.v };
        }
    };
}

or new mixin strategy for more conventional mixin strategies:

pub const Vec2 = struct {
    v: @Vector(2, u32),
    shared: Shared(@This()),
    pub fn init(xs: u32, ys: u32) Vec2 {
        return .{ .v = .{ xs, ys } };
    }
};

pub const Vec3 = struct {
    v: @Vector(3, u32),
    shared: Shared(@This()),
    pub fn init(xs: u32, ys: u32, zs: u32) Vec2 {
        return .{ .v = .{ xs, ys, zs } };
    }
};

fn Shared(comptime T: type) type {
    return struct {
        fn add(a_shared: *const @This(), b: *const T) T {
            const a: *T = @fieldParentPtr("shared", a_shared);
            return .{ .v = a.v + b.v };
        }
        fn sub(a_shared: *const @This(), b: *const T) T {
            const a: *T = @fieldParentPtr("shared", a_shared);
            return .{ .v = a.v - b.v };
        }
    };
}