Move Intermediate Dynamic-dispatch OUT; the name is obtained by applying the infamous middle-out algorithm on the word middle-out.

1. Include midout.h in the source code and mark removable regions with MIDOUT_BEGIN and MIDOUT_END.
2. Compile the whole project with macro MIDOUT_PROFILING defined.
3. Execute the program on possible inputs to gather trace information on used blocks. The trace would be written to midout_trace.<pid> by default, and this output name can be changed via the environment variable MIDOUT_OUTPUT.
4. Generate midout header file by ./gen_header.py.
5. Recompile the project and ensure that the header generated in the previous step is included before midout.h. Remember to enable LTO.
6. Now all the blocks marked in step 1 that are not executed in step 3 have been removed from the final executable; if any of them is indeed used at runtime, a trap would be triggered.

See run.sh for a concrete example.