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/tcannot_borrow.nim

@@ -0,0 +1,18 @@
+discard """
+  errormsg: "cannot borrow"
+  nimout: '''tcannot_borrow.nim(16, 7) Error: cannot borrow meh; what it borrows from is potentially mutated
+tcannot_borrow.nim(17, 3) the mutation is here
+tcannot_borrow.nim(16, 7) is the statement that connected the mutation to the parameter'''
+  line: 16
+"""
+
+{.experimental: "views".}
+
+type
+  Foo = object
+    field: string
+
+proc dangerous(s: var seq[Foo]) =
+  let meh: lent Foo = s[0]
+  s.setLen 0
+  echo meh.field

tests/views/tsplit_into_openarray.nim

@@ -0,0 +1,37 @@
+discard """
+  output: '''asdf
+231
+'''
+  cmd: "nim c --gc:arc -d:useMalloc -g $file"
+  valgrind: true
+"""
+
+{.experimental: "views".}
+
+const
+  Whitespace = {' ', '\t', '\n', '\r'}
+
+iterator split*(s: string, seps: set[char] = Whitespace,
+                maxsplit: int = -1): openArray[char] =
+  var last = 0
+  var splits = maxsplit
+
+  while last <= len(s):
+    var first = last
+    while last < len(s) and s[last] notin seps:
+      inc(last)
+    if splits == 0: last = len(s)
+    yield toOpenArray(s, first, last-1)
+    if splits == 0: break
+    dec(splits)
+    inc(last)
+
+proc `$`(x: openArray[char]): string =
+  result = newString(x.len)
+  for i in 0..<x.len: result[i] = x[i]
+
+proc main() =
+  for x in split("asdf 231"):
+    echo x
+
+main()

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])