Also simplify project structure. The executable is now produced in
the root directory. Work in progress.

The "Enter" instruction has been removed and the frame pointer is
now saved by the "Call" instruction.

The "Return" instruction now takes the number of function input
parameters as the second operand. It's used to return the output
values at the correct place in the caller frame, no matter the
number of input parameters.

The tests and the code generator have been updated accordingly.

* codegen: add a bytecode generator
* cleaning scanner, parser and vm1.