rpc: Allow named and positional arguments to be used together

It's nice to be able to use named options and positional arguments together.

Most shell tools accept both, and python functions combine options and
arguments allowing them to be passed with even more flexibility. This change
adds support for python's approach so as a motivating example:

    bitcoin-cli -named createwallet wallet_name=mywallet load_on_startup=1

Can be shortened to:

    bitcoin-cli -named createwallet mywallet load_on_startup=1

JSON-RPC standard doesn't have a convention for passing named and positional
parameters together, so this implementation makes one up and interprets any
unused "args" named parameter as a positional parameter array.

test/functional/test_framework/test_node.py

@@ -719,7 +719,6 @@ class TestNodeCLI():
         """Run bitcoin-cli command. Deserializes returned string as python object."""
         pos_args = [arg_to_cli(arg) for arg in args]
         named_args = [str(key) + "=" + arg_to_cli(value) for (key, value) in kwargs.items()]
-        assert not (pos_args and named_args), "Cannot use positional arguments and named arguments in the same bitcoin-cli call"
         p_args = [self.binary, "-datadir=" + self.datadir] + self.options
         if named_args:
             p_args += ["-named"]