Free the AST cache time after time

[XVincentX]

I am not really sure about this and probably it should be verified but -
The refract output is generally quite big and it can blow up the memory as you create new documents.

The cache is stored indefinitely (even if you close the documents, thous you do not want to work on it anymore) - even for untitled documents.

Verify and eventually prune the stale elements (probably every time a document gets closed)