Implemented mSlice on the VM allowing toOpenArray to work at compile time. (#20586)

* Implemented opcSlice to make 'toOpenArray' work on the VM

* Added nkOpenArray for VM to reduce bodgeness

* Fixed range issues and erraneous comments

* Range check correctly for openArrays in opcLdArr

* Inverted logic for ldArr checking

* vm now supports slicing strings

* Added string tests

* Removed usage of 'nkOpenArray' and redundant operations

* Refactored vmSlice implementation, removing redundant and incorrect code

* Made tuples go throw opcWrObj for field assignment

* All strkinds should be considered for openarrays

tests/vm/topenarrays.nim

@@ -0,0 +1,38 @@
+proc mutate(a: var openarray[int]) =
+  var i = 0
+  for x in a.mitems:
+    x = i
+    inc i
+
+proc mutate(a: var openarray[char]) =
+  var i = 1
+  for ch in a.mitems:
+    ch = 'a'
+
+
+static:
+  var a = [10, 20, 30]
+  assert a.toOpenArray(1, 2).len == 2
+
+  mutate(a)
+  assert a.toOpenArray(0, 2) == [0, 1, 2]
+  assert a.toOpenArray(0, 0) == [0]
+  assert a.toOpenArray(1, 2) == [1, 2]
+  assert "Hello".toOpenArray(1, 4) == "ello"
+  var str = "Hello"
+  str.toOpenArray(2, 4).mutate()
+  assert str.toOpenArray(0, 4).len == 5
+  assert str.toOpenArray(0, 0).len == 1
+  assert str.toOpenArray(0, 0).high == 0
+  assert str == "Heaaa"
+  assert str.toOpenArray(0, 4) == "Heaaa"
+
+  var arr: array[3..4, int] = [1, 2]
+  assert arr.toOpenArray(3, 4) == [1, 2]
+  assert arr.toOpenArray(3, 4).len == 2
+  assert arr.toOpenArray(3, 3).high == 0
+
+  assert arr.toOpenArray(3, 4).toOpenArray(0, 0) == [1]
+
+
+