rename some types, fields, functions, and variables related to common types

[cdisselkoen]

Description of changes

On the heels of #1059, and following this comment on #1060, this PR renames a bunch of other types, fields, functions, and variables related to common types.

• Explicitly use "common type" instead of "typedef" where it makes sense, following our docs
• Distinguish between "type_defs" (definitions of common types) and "type_refs" (references to common types)
• Make a couple field names more concise (to change e.g. self.type_defs.type_defs into self.common_types.defs)

I believe that none of the changes here are breaking, or even visible outside the cedar-policy-validator crate. The field renames are in structs that don't impl Serialize or Deserialize, and the types and functions are crate-private. Happy to be corrected if I'm wrong.

Issue #, if available

Checklist for requesting a review

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

• 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):

• Does not update the CHANGELOG because my change does not significantly impact released code.

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

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