First files for the Nit virtual machine
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.

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

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