assembler: start refactoring/rewriting tokeniser

assembler/src/error.rs

@@ -59,12 +59,18 @@ pub enum AssembleErrorKind {
     IO(std::io::Error),
     /// Errors emitted from the [`Tokeniser`].
     Tokenise(TokeniserError),
+    /// Returned for code where the functionality has not yet been implemented but we
+    /// don't want the program to panic.
+    Unimplemented(String),
 }
 
 impl Display for AssembleErrorKind {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             Self::Tokenise(why) => write!(f, "tokeniser error: {why}"),
+            Self::Unimplemented(why) => write!(f, "used unimplemented feature: {why}"),
+            Self::IO(why) => write!(f, "problem occurred with I/O: {why}"),
+            #[expect(unreachable_patterns)]
             _ => write!(
                 f,
                 "unhandled error type in Display implementation! See error.rs!"