implements higher-order inline iterators and return type inference for iterators

tests/iter/tchainediterators.nim

@@ -0,0 +1,38 @@
+discard """
+  output: '''16
+32
+48
+64
+128
+192
+'''
+"""
+
+iterator gaz(it: iterator{.inline.}): type(it) =
+  for x in it:
+    yield x*2
+
+iterator baz(it: iterator{.inline.}): auto =
+  for x in gaz(it):
+    yield x*2
+
+type T1 = auto
+
+iterator bar(it: iterator: T1{.inline.}): T1 =
+  for x in baz(it):
+    yield x*2
+
+iterator foo[T](x: iterator: T{.inline.}): T =
+  for e in bar(x):
+    yield e*2
+
+var s = @[1, 2, 3]
+
+# pass an interator several levels deep:
+for x in s.items.foo:
+  echo x
+
+# use some complex iterator as an input for another one:
+for x in s.items.baz.foo:
+  echo x
+