py/runtime: Support importing a method from an instance.

This change follows CPython behaviour, allowing use of:

    from instance import method

to import a bound method from a class instance, eg registered via
setting `sys.modules["instance"] = instance`.

Admittedly this is probably a very rarely used pattern in Python, but it
resolves a long standing comment about whether or not this is actually
possible (it turns out it is possible!).  A test is added to show how it
works.

The main reason for this change is to fix a problem with imports in the
webassembly port: prior to this fix, it was not possible to do `from
js_module import function`, where `js_module` is a JavaScript object
registered to be visible to Python through the webassembly API function
`registerJsModule(js_module)`.  But now with this fix that is possible.

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

tests/basics/import_instance_method.py

@@ -0,0 +1,38 @@
+# Test importing a method from a class instance.
+# This is not a common thing to do, but ensures MicroPython has the same semantics as CPython.
+
+import sys
+
+if not hasattr(sys, "modules"):
+    print("SKIP")
+    raise SystemExit
+
+
+class A:
+    def __init__(self, value):
+        self.value = value
+
+    def meth(self):
+        return self.value
+
+    def meth_with_arg(self, a):
+        return [self.value, a]
+
+
+# Register a class instance as the module "mod".
+sys.modules["mod"] = A(1)
+
+# Try importing it as a module.
+import mod
+
+print(mod.meth())
+print(mod.meth_with_arg(2))
+
+# Change the module.
+sys.modules["mod"] = A(3)
+
+# Try importing it using "from ... import".
+from mod import meth, meth_with_arg
+
+print(meth())
+print(meth_with_arg(4))