Add persistent state for interactive mode

[Hoxmot]

Currently, the state used in the interactive mode works only during a single interpretation. I can't declare a variable in the first line and call it in a second line:

>> (int x 42)

>> x

The above code throws an exception.

There's a workaround, where I try to keep one parenthesis open, but it's rather ugly:

>> (int x 42
>> ) x

I should add a persistent state for the interactive mode so that it's kept when the interpreter runs.