A lot of improvements

**End-user features:**
* More readable debug mode inspired by javap.
* New opcode NOP for `no operation'.
* No more memory leaks.
* Interactive commands now work properly no matter what input is used.
* New options for the VM.
* Instruction mnemonic names are now correct English.
* More errors are reported in a human readable form.
* Updated user manual.

**Internal design and implementation:**
* Introducing a very simple semantic analysis stage before running the code.
* The VM is no longer limited to 10000 LOC thanks to left recursion and memory use is reduced (credits Q. Campos and J. Mangue).
* Consistent coding style (K&R C).
* Dedicated `main.c' file.
* Dedicated `opcode' type fully compatible with the old implementation.
* Reduced visibility of global variables if needed.
* Doxygen and classic doc comments added (with a Makefile rule to check Doxygen doc).