Update InterpBlock and InterpExprs

[sonmarcho]

Following a chat: we should update InterpBlock to use InterpExprs, and update InterpExprs to return a (seq<Value>, InterpResult<()>).

The sequence of values returned by InterpExprs would contain the values computed for all the expressions which successfully evaluated:

• if the result is Success its length would be |es|
• if the result is Fail its length would be < |es|

This would allow us to check those values afterwards. For instance, even if some expression failed to evaluate, we could check that all the expressions before evaluated to (), and return the proper error if it was not the case. This should also work well once we add exceptions.