Add optional slice and map support to protobuf

Specifying // +protobuf.nullable=true on a Go type that is an alias of a
map or slice will generate a synthetic protobuf message with the type
name that will serialize to the wire in a way that allows the difference
between empty and nil to be recorded.

For instance:

    // +protobuf.nullable=true
    types OptionalMap map[string]string

will create the following message:

    message OptionalMap {
      map<string, string> Items = 1
    }

and generate marshallers that use the presence of OptionalMap to
determine whether the map is nil (rather than Items, which protobuf
provides no way to delineate between empty and nil).

cmd/libs/go2idl/go-to-protobuf/protobuf/package.go

@@ -75,6 +75,10 @@ type protobufPackage struct {
 	// A list of names that this package exports
 	LocalNames map[string]struct{}
 
+	// A list of type names in this package that will need marshaller rewriting
+	// to remove synthetic protobuf fields.
+	OptionalTypeNames map[string]struct{}
+
 	// A list of struct tags to generate onto named struct fields
 	StructTags map[string]map[string]string
 
@@ -110,7 +114,9 @@ func (p *protobufPackage) filterFunc(c *generator.Context, t *types.Type) bool {
 	case types.Builtin:
 		return false
 	case types.Alias:
-		return false
+		if !isOptionalAlias(t) {
+			return false
+		}
 	case types.Slice, types.Array, types.Map:
 		return false
 	case types.Pointer:
@@ -128,6 +134,11 @@ func (p *protobufPackage) HasGoType(name string) bool {
 	return ok
 }
 
+func (p *protobufPackage) OptionalTypeName(name string) bool {
+	_, ok := p.OptionalTypeNames[name]
+	return ok
+}
+
 func (p *protobufPackage) ExtractGeneratedType(t *ast.TypeSpec) bool {
 	if !p.HasGoType(t.Name.Name) {
 		return false