dsl/types: add String+Underlying methods to Struct

Author: quasilyte

dsl/types/type_impl.go

@@ -8,8 +8,10 @@ func (*Array) String() string     { return "" }
 func (*Slice) String() string     { return "" }
 func (*Pointer) String() string   { return "" }
 func (*Interface) String() string { return "" }
+func (*Struct) String() string    { return "" }
 
 func (*Array) Underlying() Type     { return nil }
 func (*Slice) Underlying() Type     { return nil }
 func (*Pointer) Underlying() Type   { return nil }
 func (*Interface) Underlying() Type { return nil }
+func (*Struct) Underlying() Type    { return nil }