First files for the Nit virtual machine

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
A virtual table is now constructed for each MClass
The virtual table is constructed according to the specifications.

The perfect hashtable is at negative offsets.
It contains pointer to blocks of (id, methods) which
are at positive offsets of the virtual table.

Also change the numbering in perfect_hashing (start at 1 instead of 0)

Signed-off-by: Julien Pagès <julien.projet@gmail.com>
Subtyping test with perfect hashing in the Nit virtual machine

Signed-off-by: Julien Pagès <julien.projet@gmail.com>