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/ccgbugs/tviews1.nim

@@ -1,27 +0,0 @@
-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])