Flush stdout after PUTS and OUT traps

[dxrcy]

Fixes #24
Always flush stdout after PUTS or OUT trap call.

This could hypothetically affect performance for programs with a large number of consecutive OUT operations (though I don't think this matters).
A way to solve this would be to flush stdout before IN trap reads stdin. Since this is the only blocking instruction, and stdout is automatically flushed when the process unwinds (triggered by HALT or a panic), this should work for most cases (std::process::abort will not cause unwinding, but if this program has to abort, it has bigger problems anyway). Regardless, with this alternative method, the problem will still occur if a large (or infinite) loop is executed before stdout can be flushed.

[rozukke]

👍 Looks good, don't think performance is too big of a deal here