Add compile caching for Interpreter

[matajoh]

At the moment the interpreter restarts compilation for each query. It should instead cache the step immediately before unification so that everything but the input and query are already good to go.

Notes

• This will not work for more complex queries, or rather they may still need to pass through compilation.
• Input will need to be parsed as well (this will need to be done by the standalone JSON parser from json built-ins #60)

[matajoh]

A step towards this has been done in 0.4.0, with intermediate caching of all documents and input terms. There is still more that could potentially be done, however.