py/objdict: Implement bool and len unary ops for dict views.

Currently, dict views (eg `dict.keys()`, `dict.values()`) do not implement
the `bool` or `len` unary operations.  That may seem like a reasonable
omission for MicroPython to keep code size down, but it actually leads to
silently incorrect bool operations, because by default things are true.

Eg we currently have:

    >>> bool(dict().keys())
    True

which is wrong, it should be `False` because the dict is empty.

This commit implements `bool` and `len` unary operations on dict views by
simply delegating to the existing dict unary op function.

Fixes issue #12385.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/dict_views.py

@@ -6,6 +6,11 @@ for m in d.items, d.values, d.keys:
 # print a view with more than one item
 print({1:1, 2:1}.values())
 
+# `bool` and `len` unary ops
+for d in ({}, {1: 2}, {1: 2, 3: 4}):
+    for op in (bool, len):
+        print(op(d.keys()), op(d.values()), op(d.items()))
+
 # unsupported binary op on a dict values view
 try:
     {1:1}.values() + 1