move compat code into persist_compat.go

MERGE REQUEST

Overview

A couple of structs and functions are still defined in files.go that are
misleading because they are only used in the compat process to upgrade
an old file format. Some documentation has been added to express this
fact, and all of the compat-only code has been moved to
persist_compat.go.

I marked this MR as 'documentation', because the changes are purely
cosmetic/organizational.

Documentation Changes

Some old structs have been marked as old structs in their docstrings.

Modules and Packages Impacted

Renter

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• Pipeline Passing (this is for NDFs where tests were run in a loop)
• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features