replace x - (x // y) * y with x % y in divmod

`evaluable.divmod(x, y)` returns `x - (x // y) * y` as remainder. There are
three issues with this:

*   When calling `evaluable.div`, `evaluable.mod` *and* `evaluable.divmod`
    we're doing extra work. This can be solved by changing the implementation
    of `evaluable.mod`, but:

*   It appears to be cheaper to compute the modulo using `numpy.mod` on x86
    since the remainder is a byproduct of the
    [div](https://www.felixcloutier.com/x86/div) instruction.

*   `evaluable.Mod` (note the capital) has an implementation for the integer
    bounds and the remainder of `evaluable.divmod` has not.

This patch changes the implementation of the remainder of `evaluable.divmod` to
`evaluable.Mod`.

nutils/evaluable.py

@@ -4773,9 +4773,8 @@ def ln(x):
 
 
 def divmod(x, y):
-    div = FloorDivide(*_numpy_align(x, y))
-    mod = x - div * y
-    return div, mod
+    x, y = _numpy_align(x, y)
+    return FloorDivide(x, y), Mod(x, y)
 
 
 def mod(arg1, arg2):