Implement parser & formater for 'TraceIfFalse'

Interestingly enough, chumsky seems to fail when given a 'choice' with
  more than 25 elements. That's why this commit groups together some of
  the choices as another nested 'choice'.

crates/aiken-lang/src/parser/lexer.rs

@@ -35,10 +35,13 @@ pub fn lexer() -> impl Parser<char, Vec<(Token, Span)>, Error = ParseError> {
         just('.').to(Token::Dot),
         just("!=").to(Token::NotEqual),
         just('!').to(Token::Bang),
-        just("<=").to(Token::LessEqual),
-        just('<').to(Token::Less),
-        just(">=").to(Token::GreaterEqual),
-        just('>').to(Token::Greater),
+        just('?').to(Token::Question),
+        choice((
+            just("<=").to(Token::LessEqual),
+            just('<').to(Token::Less),
+            just(">=").to(Token::GreaterEqual),
+            just('>').to(Token::Greater),
+        )),
         just('+').to(Token::Plus),
         just("->").to(Token::RArrow),
         just('-').to(Token::Minus),

crates/aiken-lang/src/parser/token.rs

@@ -39,8 +39,9 @@ pub enum Token {
     // Other Punctuation
     Colon,
     Comma,
-    Hash, // '#'
-    Bang, // '!'
+    Hash,     // '#'
+    Bang,     // '!'
+    Question, // '?'
     Equal,
     EqualEqual, // '=='
     NotEqual,   // '!='
@@ -125,6 +126,7 @@ impl fmt::Display for Token {
             Token::Hash => "#",
             Token::Bang => "!",
             Token::Equal => "=",
+            Token::Question => "?",
             Token::EqualEqual => "==",
             Token::NotEqual => "!=",
             Token::Vbar => "|",