Fix operations on string as openarray in VM. (#6257)

* fix openarray.len for string as openArray in VM.
* fix openarray[idx] for string as openArray in VM.
* fix openarray[idx]=val for string as openArray in VM.
* add tests for passing string as openArray in VM.
* fix issue with NimNode.len
NimNode.len was also returning len for string literals.
also fix logic bug in if/elif chain.

tests/vm/tstring_openarray.nim

@@ -0,0 +1,34 @@
+
+# tests various bug when passing string to openArray argument in VM.
+# bug #6086
+proc map*[T, S](data: openArray[T], op: proc (x: T): S {.closure.}):
+                                                            seq[S]{.inline.} =
+# map inlined from sequtils
+  newSeq(result, data.len)
+  for i in 0..data.len-1: result[i] = op(data[i])
+
+
+proc set_all[T](s: var openArray[T]; val: T) =
+  for i in 0..<s.len:
+    s[i] = val
+
+proc test() =
+    var a0 = "hello_world"
+    var a1 = [1,2,3,4,5,6,7,8,9]
+    var a2 = @[1,2,3,4,5,6,7,8,9]
+    a0.set_all('i')
+    a1.set_all(4)
+    a2.set_all(4)
+    doAssert a0 == "iiiiiiiiiii"
+    doAssert a1 == [4,4,4,4,4,4,4,4,4]
+    doAssert a2 == @[4,4,4,4,4,4,4,4,4]
+
+const constval0 = "hello".map(proc(x: char): char = x)
+const constval1 = [1,2,3,4].map(proc(x: int): int = x)
+
+doAssert("hello".map(proc(x: char): char = x) == constval0)
+doAssert([1,2,3,4].map(proc(x: int): int = x) == constval1)
+
+test()
+static:
+    test()