selfhost: add @[Drop] attribute for user-defined structs

- ast.bux: add isDrop field to Decl
- parser.bux: parse @[Drop] attribute and attach to struct Decls
- hir_lower.bux: auto-drop now looks for TypeName_Drop for any type
- Test: _test_drop_user verifies Buffer_Drop is auto-called

src/ast.bux

@@ -274,6 +274,7 @@ struct Decl {
     isPublic: bool,
     isAsync: bool,
     isChecked: int,          // @[Checked] attribute (0/1)
+    isDrop: int,             // @[Drop] attribute (0/1)
     // Names
     strValue: String,        // decl name
     strValue2: String,       // interface name, dll name, module path