Fix formatter handling of blank lines

[khieta]

Description of changes

The cause of #862 was the call to remove_empty_lines, which removed empty lines from the formatted policy -- including those in string literals.

Attempt 1: It was simple enough to remove the remove_empty_lines call, but this makes formatted policies ugly since the current code introduces extra newlines around comments. I imagine the reason remove_empty_lines was originally introduced was to handle these extra newlines.

Attempt 2: I tried to get rid of the extra newlines by adjusting how RcDocs are constructed, but that ended up being a pain in the butt.

Attempt 3 (current PR): I added a new function remove_empty_lines_safe, which does some light parsing to avoid removing newlines in strings.

Summary

• Added the remove_empty_lines_safe function ← the important change
• Switched to using RcDoc::text in cases where the call to to_string was unnecessary
• Added comments in some parts of the code that are subtle.

The last two changes are just edits I made while getting familiar with the codebase. I think they're harmless.

We had previously discussed not using RcDoc::text or RcDoc::as_string for string parsing. But I think it’s actually ok: newlines are used in the tests. I think the way to interpret the "must not contain line breaks" guidance is that indentation will not be handled correctly when you do this, which is actually what we want for string literals.

Issue #, if available

Resolves #862

Checklist for requesting a review

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

• A bug fix or other functionality change requiring a patch to cedar-policy.

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):

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

[khieta]

Marking as ready-for-review since the code is in a reviewable state & CI is passing. I plan on running the formatter fuzzing targets later today -- will add a comment here with the results.

[john-h-kastner-aws]

Look pretty good.

[khieta]

Ran the formatter target for an hour with no issue. Ran the formatter-bytes target for a few minutes and it immediately found the issue (nice addition @john-h-kastner-aws!). I've fixed the identified bug, and now I'm letting the formatter-bytes target run some more 🙂

[shaobo-he-aws]

LGTM. Thanks a lot for fixing it.

[khieta]

Update: formatter-bytes has been running for an hour and a half with no failures, so I'm going to call this good.