Generate actions HashMap at ValidatorSchema instantiation

[tpaulus]

Description of changes

This change fixes part of the issues identified in #1285 by pulling up the expensive action inversion from CoreSchema construction to ValidatorSchema construction, which is only done once per Schema.

Checklist for requesting a review

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

• [X] A change "invisible" to users (e.g., documentation, changes to "internal" crates like cedar-policy-core, cedar-validator, etc.)

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

• [x] 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):

• [X] Does not require updates because my change does not impact the Cedar formal model or DRT infrastructure.

I confirm that docs.cedarpolicy.com (choose one, and delete the other options):

• [X] Does not require updates because my change does not impact the Cedar language specification.