Use PEP 448 for better unquoted expressions

With Python 3.5, we can have an arbitrary number of unpackings. We can
use this to emit cleaner expressions.

Falls back to legacy behaviour for older Python versions.

hy/compiler.py

@@ -699,28 +699,31 @@ def _render_quoted_form(self, form, level):
         imports = set([name])
 
         if isinstance(form, (HyList, HyDict, HySet)):
-            if not form:
-                contents = HyList()
-            else:
-                # If there are arguments, they can be spliced
-                # so we build a sum...
-                contents = HyExpression([HySymbol("+"), HyList()])
-
+            contents = HyList()
             for x in form:
                 f_imports, f_contents, splice = self._render_quoted_form(x,
                                                                          level)
                 imports.update(f_imports)
                 if splice:
-                    to_add = HyExpression([
-                        HySymbol("list"),
-                        HyExpression([HySymbol("or"), f_contents, HyList()])])
+                    # On Python 3.5 and newer, leverage PEP 448 to generate
+                    # expression using tuple unpacking instead of addition.
+                    #
+                    # Should be marginally faster too.
+                    contents.append(HyExpression([
+                        HySymbol("unpack-iterable" if PY35 else "list"),
+                        HyExpression([HySymbol("or"), f_contents, HyList()])
+                    ]))
                 else:
-                    to_add = HyList([f_contents])
+                    contents.append(f_contents if PY35
+                                    else HyList([f_contents]))
 
-                contents.append(to_add)
+            if not PY35:
+                # Without Python 3.5's unpacking improvements, we need
+                # to concatenate the expression manually
+                contents = HyExpression([HySymbol("sum"), contents, HyList()])
 
-            return imports, HyExpression([HySymbol(name),
-                                          contents]).replace(form), False
+            expr = HyExpression([HySymbol(name), contents])
+            return imports, expr.replace(form), False
 
         elif isinstance(form, HyCons):
             ret = HyExpression([HySymbol(name)])