Separate `parse` method and improve...

[danthedeckie]

Description

• Split 'parse' stage to separate method
• allow previously_parsed in 'eval'
• Warn if multiple expressions.

This allows AST caching as in #85, #60 etc - but the caching should be done outside of the evaluator.

The warning about multiple expressions is something I've wanted for a while.

TODO

• add to docs.

Pre-approval checklist (for submitter)

Please complete these steps

• [x] Passes tests
• [x] New tests for additional features or changed functionality