Add support for new schema syntax

[khieta]

Issue #, if available:

Resolves#130

Description of changes:

• Added support for the new schema syntax
• Moved the tests from interface.rs to a separate tests.rs file
• Removed some unused code from the Rust interface
• Updated integration testing code to use the tests in https://github.com/cedar-policy/cedar-integration-tests

[cdisselkoen]

Returns the string Success on success, but on the Java side, Schema.parse() seems to expect it to return the schema text?

[khieta]

Ah, that is indeed not doing the right thing (and testing didn't catch it since we don't have any tests that do something with a parsed schema). Will fix in next commit.

[cdisselkoen]

Curious why return the schema contents? Java already had a string containing the schema contents, I think?

[khieta]

Yes, Java already has the string -- the only new information "returned" by the parsing function is whether an exception was thrown or not. Looking at it again, it likely saves some time not to pass back the string, so I'll revert to that.

[khieta]

It turned out to not be too difficult to add support for the new integration tests, so I added that change to this PR.

[andrewmwells-amazon]

[nit] Can we keep the comment explaining why 4c isn't run?

[khieta]

I deleted it because that test was not moved over to https://github.com/cedar-policy/cedar-integration-tests -- do you think we should maintain this test in our integration test repo even if it's not used?