Remove closure borrow in 'FromForm' derive.

The codegen for field validations previously included a closure that
could potentially partially borrow a 'Copy' field of the context
structure. To prevent this, 'let'-assign the field before the closure is
created, and use the assignment inside of the closure.

core/codegen/src/derive/form_field.rs

@@ -368,7 +368,8 @@ pub fn validators<'v>(field: Field<'v>) -> Result<impl Iterator<Item = syn::Expr
                     _ => Ok(()),
                 };
 
-                __result.map_err(|__e| match #name_opt {
+                let __e_name = #name_opt;
+                __result.map_err(|__e| match __e_name {
                     Some(__name) => __e.with_name(__name),
                     None => __e
                 })