Extract preprocessing from tokenizer

bin/test.wasm

@@ -1,4 +1,4 @@
-#DEFINE
+#DEFINE LETTER_O 'o'
 
 addi $10 $-5 %A;
 subi %A $2 %B;
@@ -31,7 +31,7 @@ int PRINT_CHAR;
 seti %A $108; # l
 int PRINT_CHAR;
 int PRINT_CHAR;
-seti %A $111; # o
+seti %A LETTER_O;
 int PRINT_CHAR;
 
 seti %A $32; # space
@@ -105,9 +105,6 @@ seti %B $10;
 int $5;
 int $3;
 
-# Demonstrate string literals
-"Hello world!";
-
 exit;
 
 noop_function: