Rewrite main logic
Rewrite message logic
Simplify teensy_open
Don't use msvcrt's broken printf/scanf functions

Define __USE_MINGW_ANSI_STDIO to use MinGW's alternative implementations,
which understands all C99 specifiers. Get rid of some workarounds.
Rename project to tesy
Don't include windows.h in teensy.h

Treat the OVERLAPPED ov member of teensy_handle_t as a pointer instead
of embedding the struct. Could also have copied the OVERLAPPED declaration
from windows.h, but this looks cleaner despite an extra malloc.