borrow checking (#15282)

* refactoring: move procs to typeallowed.nim
* frontend preparations for first class openArray support
* prepare the code generator for first class openArray
* code generation for first class openArray; WIP
* code generation for open arrays, progress
* added isViewType proc
* preparations for borrow checking
* added borrow checking to the front end

tests/ccgbugs/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])