py: Add optional support for 2-argument version of built-in next().

Configurable via MICROPY_PY_BUILTINS_NEXT2, disabled by default.

tests/basics/builtin_next_arg2.py

@@ -0,0 +1,34 @@
+# test next(iter, default)
+
+try:
+    next(iter([]), 42)
+except TypeError:  # 2-argument version not supported
+    print('SKIP')
+    raise SystemExit
+
+print(next(iter([]), 42))
+print(next(iter(range(0)), 42))
+print(next((x for x in [0] if x == 1), 43))
+
+def gen():
+    yield 1
+    yield 2
+
+g = gen()
+print(next(g, 42))
+print(next(g, 43))
+print(next(g, 44))
+
+class Gen:
+    def __init__(self):
+        self.b = False
+
+    def __next__(self):
+        if self.b:
+            raise StopIteration
+        self.b = True
+        return self.b
+
+g = Gen()
+print(next(g, 44))
+print(next(g, 45))