rename and flatten frontend

[cdisselkoen]

Description of changes

• Renames frontend module to ffi
• Flattens the frontend module. Instead of seeing submodules is_authorized, validate, and utils, everything is exposed in the flat ffi namespace. (Internally we still have separate files for is_authorized, validate, and utils.
• Also removes the InterfaceResult structure which is dead after 760's changes but for validation APIs #793

Issue #, if available

As suggested in discussion on #757

Checklist for requesting a review

The change in this PR is (choose one, and delete the other options):

• A breaking change requiring a major version bump to cedar-policy (e.g., changes to the signature of an existing API).

I confirm that this PR (choose one, and delete the other options):

• Updates the "Unreleased" section of the CHANGELOG with a description of my change (required for major/minor version bumps).

I confirm that cedar-spec (choose one, and delete the other options):

• Requires updates, and I have made / will make these updates myself. (Please include in your description a timeline or link to the relevant PR in cedar-spec, and how you have tested that your updates are correct.)

[khieta]

Confirming that the current CI failures are expected: downstream DRT code is broken by this PR, downstream Java code is broken by this PR (also currently in a broken state due to #760 and #793), and the semver checks are failing because this PR is indeed a breaking change.