nat: make PortMap and PortSet aliases

The `PortSet` and `PortMap` types don't carry a real meaning on their
own. Their purpose is defined at locations where they're used, such as
`Container.ExposedPorts` or `Container.PortBindings`.

Their documentation also shows this;

    // PortSet is a collection of structs indexed by [PortRangeProto].
    // PortMap is a collection of [PortBinding] indexed by [PortRangeProto].

Which, almost literally, describes what a map is. Substituting the types
for their implementation doesn't lose any meaning;

    // map[PortRangeProto]struct{} is a collection of structs indexed by [PortRangeProto].
    // map[PortRangeProto][]PortBinding is a collection of [PortBinding] indexed by [PortRangeProto].

Neither type has any special handling connected to them (no methods or
otherwise), so they are just maps with a fancy name attached. Worse,
using the extra indirect induces cognitive load; it's not clear from
the type that it's "just" a map, indexed by `PortBinding`, and it's
not clear what (kind of) values are in the map.

There are cases where having a type defined can add value to provide
a more in-depth description of their intent, but (as shown above) even
that is not the case here.

I'm considering these types to be premature abstraction with no good
value. As they are a "straight" copy of a map with the same signature,
replacing these types preserves backward compatibility; existing code
can assign either a `PortSet` or `PortMap`, or a `map[PortRangeProto]..`
with the same signature, but we can deprecate the types to give users
a nudge to use a regular map instead. The following shows that they are
interchangeable;

	type config1 struct {
		ExposedPorts container.PortSet
		PortBindings container.PortMap
	}
	type config2 struct {
		ExposedPorts map[container.PortRangeProto]struct{}
		PortBindings map[container.PortRangeProto][]container.PortBinding
	}

	var (
		ports         map[container.PortRangeProto]struct{}
		portBindings  map[container.PortRangeProto][]container.PortBinding
		ports2        container.PortSet
		portBindings2 container.PortMap
	)
	_ = config1{
		ExposedPorts: ports,
		PortBindings: portBindings,
	}
	_ = config1{
		ExposedPorts: ports2,
		PortBindings: portBindings2,
	}
	_ = config2{
		ExposedPorts: ports,
		PortBindings: portBindings,
	}
	_ = config2{
		ExposedPorts: ports2,
		PortBindings: portBindings2,
	}

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

nat/nat.go

@@ -18,10 +18,10 @@ type PortBinding struct {
 }
 
 // PortMap is a collection of PortBinding indexed by Port
-type PortMap map[Port][]PortBinding
+type PortMap = map[Port][]PortBinding
 
 // PortSet is a collection of structs indexed by Port
-type PortSet map[Port]struct{}
+type PortSet = map[Port]struct{}
 
 // Port is a string containing port number and protocol in the format "80/tcp"
 type Port string