hdl.ir: rework named port handling for Instances.

The main purpose of this rework is cleanup, to avoid specifying
the direction of input ports in an implicit, ad-hoc way using
the named ports and ports dictionaries.

While working on this I realized that output ports can be connected
to anything that is valid on LHS, so this is now supported too.

nmigen/back/rtlil.py

@@ -669,7 +669,7 @@ class _StatementCompiler(xfrm.StatementVisitor):
 def convert_fragment(builder, fragment, hierarchy):
     if isinstance(fragment, ir.Instance):
         port_map = OrderedDict()
-        for port_name, value in fragment.named_ports.items():
+        for port_name, (value, dir) in fragment.named_ports.items():
             port_map["\\{}".format(port_name)] = value
 
         if fragment.type[0] == "$":