checker: fix assigning array slice (fix #19120)

[yuyi98]

This PR fix assigning array slice (fix #19120).

• Fix assigning array slice.
• mut a := arr[..i] -> mut a := arr[..i].clone() if left var or right var is mut and not inside unsafe.
• Add test.

fn main() {
	xs := [1, 2, 3, 4, 5, 6, 7, 8]

	mut s := xs[1..]

	s.sort(a > b)

	println(s)
	assert s == [8, 7, 6, 5, 4, 3, 2]
	println(xs)
	assert xs == [1, 2, 3, 4, 5, 6, 7, 8]
}

PS D:\Test\v\tt1> v run .
tt1.v:4:13: notice: an implicit clone of the slice was done here
    2 |     xs := [1, 2, 3, 4, 5, 6, 7, 8]
    3 | 
    4 |     mut s := xs[1..]
      |                ~~~~~
    5 | 
    6 |     s.sort(a > b)
Details: tt1.v:4:13: details: To silence this notice, use either an explicit `a[..].clone()`, 
or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.
    2 |     xs := [1, 2, 3, 4, 5, 6, 7, 8]
    3 |
    4 |     mut s := xs[1..]
      |                ~~~~~
    5 |
    6 |     s.sort(a > b)
[8, 7, 6, 5, 4, 3, 2]
[1, 2, 3, 4, 5, 6, 7, 8]

[spytheman]

These should not need unsafe, arr2 is immutable, but slice2 and slice3 are not mutable too -> they can not be used to make modifications to arr2.

[yuyi98]

"unsafe" here is not just about not modifying the immutable array.
Also to ensure that the left var and right arrays are not affected by their respective changes.

[spytheman]

Also to ensure that the left var and right arrays are not affected by their respective changes.

That is a valid viewpoint, now that I think about it more. It will make some programs a little more cumbersome, but clearer/less surprising indeed.