Add support for polarity (+/-) operator

[piotrszul]

Currently unary +/- operators are not supported which makes is it impossible (or difficult) use negative literals e.g x < -1 .

(FHIR R4 Exclusions)

Details

au.csiro.pathling.errors.InvalidUserInputError: Polarity operator is not supported

• -3 != 3 [** testLiteralIntegerNotEqual]
• -Patient.name.given.count() = -5 [** testPolarityPrecedence]
• Patient.name.given.count() > -3 [** testLiteralIntegerGreaterThan]
• 1 < -2 [** testLiteralIntegerLessThanFalse]
• +1 < +2 [** testLiteralIntegerLessThanPolarityTrue]
• -1 < 2 [** testLiteralIntegerLessThanPolarityFalse]
• (-1).toString() [** testNegativeIntegerLiteralToString]