Correct behavior of attribute reading in nitvm

This (small) PR allows the nitvm to pass more tests related to attributes reading.

The first commit introduces a new type and Instance which represent the initial values inside attributes.
In the second commit, the vm now throw the expected error when reading an uninitialized attribute.
The last commit implements ```isset``` expressions in the vm.

So, all tests base_attr*.nit are now corrects in the vm.