This changes the API of xfa from
XFA(array_name, prio) type element_name = INITIALIZER;
to
XFA(type, array_name, prio) element_name = INITIALIZER;
this allows forcing natural alignment of the type, fixing failing tests
on `native64`.
A couple of `stdio` backend implementations allow to check for the number of available bytes for reading before the blocking `stdio_read` is called. This helps to implement non-blocking functionalities while waiting for `stdin`.
