Make @shlExact() more ergonomic when type casting

[momumi]

Currently using @shlExact(value: T, shift: Log2T) T gets very messy when using values that aren't comptime, for example:

fn makePixel(info: c.pixel_info, r: u8, b: u8, g: u8) u32 {
    return (
          @shlExact(@intCast(u32, r), @intCast(u5, info.red.offset))
        | @shlExact(@intCast(u32, g), @intCast(u5, info.green.offset))
        | @shlExact(@intCast(u32, b), @intCast(u5, info.blue.offset))
    );
}

I think it would be much cleaner to use @shlExact(T: type, value: T, shift: T) T thus:

fn makePixel(info: c.pixel_info, r: u8, b: u8, g: u8) u32 {
    return (
          @shlExact(u32, r, info.red.offset)
        | @shlExact(u32, g, info.green.offset)
        | @shlExact(u32, b, info.blue.offset)
    );
}

It would still provide all the same safety checks, it is just way neater.

Also, it might be nice to have a matching function @shl like:

@shl(u32, r, offset) == (@intCast(u32, r) << @intCast(u5, offset))

That's kinda redundant I guess, but it would give a standard and clean way to do this cast and the symmetry between @shl and @shlExact seems nice to me.