refactor(ast/builder): use Box::new_in over .into_in (#4428)

In `AstBuilder`'s `alloc_*` methods, use `Box::new_in` instead of `.into_in`. This is more explicit, so I feel easier to understand. It may also make life easier for compiler by not requiring it to perform type coercion.

tasks/ast_codegen/src/generators/ast_builder.rs

@@ -277,7 +277,7 @@ fn generate_struct_builder_fn(ty: &RStruct, ctx: &CodegenCtx) -> TokenStream {
         endl!();
         #[inline]
         pub fn #alloc_fn_name #generic_params (self, #(#params),*) -> Box<'a, #as_type> #where_clause {
-            self.#fn_name(#(#args),*).into_in(self.allocator)
+            Box::new_in(self.#fn_name(#(#args),*), self.allocator)
         }
     }
 }