Start the foundations for big integers.

* Architecture: BigInt -> BigIntImplem (abstract, interface)
                       -> BoostBigInt (which will contain an actual boost multi precision integer).
* Using shared_ptr ensures automatic deallocation.
* The VM argument might not be needed in VirtualMachineEnvironment::newBigIntImplem
  but right now it is to raise an exception in the default VMEnv.
  That method should of course never be called in the default VMEnv
  (only from BigInt when we managed to call newBigInt()).
* We need to shrink BigInt to SmallInt.