feat(if): Support inequality operator

Sources/Expression.swift

@@ -134,11 +134,11 @@ final class AndExpression: Expression, InfixOperator, CustomStringConvertible {
 }
 
 
-final class EqualityExpression: Expression, InfixOperator, CustomStringConvertible {
+class EqualityExpression: Expression, InfixOperator, CustomStringConvertible {
   let lhs: Expression
   let rhs: Expression
 
-  init(lhs: Expression, rhs: Expression) {
+  required init(lhs: Expression, rhs: Expression) {
     self.lhs = lhs
     self.rhs = rhs
   }
@@ -170,6 +170,17 @@ final class EqualityExpression: Expression, InfixOperator, CustomStringConvertib
 }
 
 
+class InequalityExpression: EqualityExpression {
+  override var description: String {
+    return "(\(lhs) != \(rhs))"
+  }
+
+  override func evaluate(context: Context) throws -> Bool {
+    return try !super.evaluate(context: context)
+  }
+}
+
+
 func toNumber(value: Any) -> Float80? {
     if let value = value as? Float {
         return Float80(value)