ast: Add TypeDecl Node (#5102)

ast: A TypeDecl Node

This commit adds a separate AST decl type for type declarations as
opposed to using const deleclartions for this case. This is part of a
series of changes preparing the AST code base to support position
information for AST nodes.

compiler/ast/ast.go

@@ -239,9 +239,16 @@ type OpDecl struct {
 	Body   Seq      `json:"body"`
 }
 
+type TypeDecl struct {
+	Kind string      `json:"kind" unpack:""`
+	Name string      `json:"name"`
+	Type astzed.Type `json:"type"`
+}
+
 func (*ConstDecl) DeclAST() {}
 func (*FuncDecl) DeclAST()  {}
 func (*OpDecl) DeclAST()    {}
+func (*TypeDecl) DeclAST()  {}
 
 // ----------------------------------------------------------------------------
 // Operators

compiler/ast/unpack.go

@@ -76,6 +76,7 @@ var unpacker = unpack.New(
 	Top{},
 	astzed.TypeArray{},
 	astzed.TypeDef{},
+	TypeDecl{},
 	astzed.TypeEnum{},
 	astzed.TypeError{},
 	astzed.TypeMap{},