ffi: const quiche_path_event* expected to be passed to quiche_path_event* parameters

[ichordev]

The only way to get a quiche_path_event is from the return value of quiche_conn_path_event_next. It returns a const quiche_path_event*. However, all the functions that operate on a quiche_path_event take a mutable quiche_path_event* in quiche.h, meaning you have to cast away const on the pointer to use it anywhere or else you get an error. This seems to be a mistake, because in ffi.rs none of those function parameters are mut (with the exception of quiche_path_event_free). Perhaps quiche_conn_path_event_next should just return a mutable pointer?