Implemented WIP grammar import memoization

Code review checklist

[x] Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
[x] Title summarizes what is changing
[x] Commit messages are meaningful (see this for details)
[ ] Tests have been included and/or updated
[ ] Docstrings have been included and/or updated, as appropriate
[ ] Standalone docs have been updated accordingly
[ ] Changelog(s) has/have been updated, as needed (see CHANGELOG.md, no need to update for typo fixes and such).