merge: refactor Buffer handling

 * Tidy some Buffer API, keep most buffers on the stack
 * Keep spam member of struct Envelope by value
 * Kill mutt_buffer_from
 * Move buffer_new / buffer_free to static functions in pool.c
 * Add mutt_buffer_make to create a Buffer on the stack
 * Use mutt_buffer_is_empty instead of mutt_buffer_len where it makes sense