py: Implement full behaviour of dict.update(), and dict().

Add keyword args to dict.update(), and ability to take a dictionary as
argument.

dict() class constructor can now use dict.update() directly.

This patch loses fast path for dict(other_dict), but is that really
needed?  Any anyway, this idiom will now re-hash the dictionary, so is
arguably more memory efficient.

Addresses issue #647.

tests/basics/dict_construct.py

@@ -0,0 +1,16 @@
+# dict constructor
+
+d = dict()
+print(d)
+
+d = dict({1:2})
+print(d)
+
+d = dict(a=1)
+print(d)
+
+d = dict({1:2}, a=3)
+print(d[1], d['a'])
+
+d = dict([(1, 2)], a=3, b=4)
+print(d[1], d['a'], d['b'])