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/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()