Feature request: mark functions `noexcept` in C++

[rohanlean]

Hi,

first of all, many thanks for creating io_uring!

Because you seem to be using raw syscalls everywhere instead of POSIX wrappers that could be cancellation points, the functions in liburing thankfully do not throw and probably never will. Yet they are not marked noexcept, which inhibits optimisations.

I propose that all functions be marked noexcept.