Add basic EST round-trip testing

[khieta]

Issue #, if available:

#204

Description of changes:

This PR renames the pp target to roundtrip and adds two additional checks per policy:

1. Check that parsing a displayed policy using parser::parse_policy_or_template_to_est and converting to JSON produces no errors.
2. Check that converting a policy to JSON and back preserves the policy structure.

To test, I reverted cedar#601 and confirmed that the underlying bug was found quickly by check 1. Check 2 didn't turn up any bugs during an overnight run, but it seems useful(?)

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[cdisselkoen]

See also the tests and test utilities in est.rs, which define three different kinds of roundtrips involving EST, one of which is always lossless.

[cdisselkoen]

The lossy step is EST->AST or CST->AST, neither of which is included in this roundtrip I think? (I believe this takes AST input and goes AST->text->EST->JSON?) So I think we actually could check that the resulting policy matches the input

[khieta]

EST->AST would be required here because check_policy_equivalence expects two ASTs.

But it would be possible to define this function for ESTs too. I'll update this PR later to include more variants, inspired by the helper functions in est.rs.

[khieta]

Update: just looked at this again, and I think this step actually should be lossless. I assumed the translation was supposed to be lossy since DRT failed when I compare the outputs. But looking at the failure again, it looks like DRT was pointing out an actual error: cedar#618, which should be resolved by cedar#622. I'll add in the code to do the AST comparison, and test again once cedar#622 is finished.