Implement almost all functionality

Add keywords and identifiers
Add comments
Add number literals
Add strings
Add a lot of new tokens
Add (and clean up) IF etc.
Improve reporting of unexpected chars / garbage bytes
Fix bug with and improved error messages when failing to open file
Add verbose-level messages about how files are opened
Enforce that files finish with a newline
Fix chars returned not being cast to unsigned char (may conflict w/ EOF)
Return null path when no file is open, rather than crash
Unify and improve error printing slightly

Known to be missing: macro expansion, REPT blocks, EQUS expansions

src/asm/main.c

@@ -488,6 +488,7 @@ int main(int argc, char *argv[])
 
 	if (!state)
 		fatalerror("Failed to open main file!\n");
+	lexer_Init();
 	lexer_SetState(state);
 
 	nStartClock = clock();