Fix Escaped Identifiers

[halprin]

I created a new branch, fix-escaped-identifier. In that branch, I created a test in evaluate_test.go, called TestEvaluate_EscapedIdentifier that basically tests FHIR Path that contains backticks. E.g. name.`given`. Notice the backticks. name.given works, but not with the backticks around the identifier. This should also fix some of the official tests in official_test.go, specifically TestOfficial/testBasics/testEscapedIdentifier. Feel free to reach out if you have questions.