better support for slices as views (#15414)

* moved view tests to tests/views
* refactoring
* more refactorings
* better support for system.toOpenArray for first class view types

tests/views/tviews1.nim

@@ -0,0 +1,27 @@
+discard """
+  output: '''11
+22
+33
+3
+2
+3'''
+  targets: "c cpp"
+"""
+
+{.experimental: "views".}
+
+proc take(a: openArray[int]) =
+  echo a.len
+
+proc main(s: seq[int]) =
+  var x: openArray[int] = s
+  for i in 0 .. high(x):
+    echo x[i]
+  take(x)
+
+  take(x.toOpenArray(0, 1))
+  let y = x
+  take y
+
+
+main(@[11, 22, 33])