Improve 3.5+ BUILD_MAP_UNPACK...

And add build_tuple_unpack runtime test from a previous commit.

We are far from out of the woods, as there is more to do and
we've uncovered more bugs in handling this.

test/simple_source/bug35/07_build_map_unpack.py

@@ -0,0 +1,10 @@
+# Adapted from Python 3.6 trace.py
+# Bug was in handling BUID_TUPLE_UNPACK created via
+# *opts.arguments
+import argparse
+parser = argparse.ArgumentParser()
+parser.add_argument('filename', nargs='?')
+parser.add_argument('arguments', nargs=argparse.REMAINDER)
+opts = parser.parse_args(["foo", "a", "b"])
+argv = opts.filename, *opts.arguments
+assert argv == ('foo', 'a', 'b')