Implements #2154.

When unpacking tuples in var/let declarations a part of the tuple can now
be discarded using a single underscore.

tests/parser/ttupleunpack.nim

@@ -0,0 +1,18 @@
+discard """
+  file: "ttupleunpack.nim"
+  output: ""
+  exitcode: 0
+"""
+proc foo(): tuple[x, y, z: int] =
+  return (4, 2, 3)
+
+var (x, _, y) = foo()
+doAssert x == 4
+doAssert y == 3
+
+iterator bar(): tuple[x, y, z: int] =
+  yield (1,2,3)
+
+for x, y, _ in bar():
+  doAssert x == 1
+  doAssert y == 2