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

include/asm/util.h

@@ -12,6 +12,7 @@
 #include <stdint.h>
 
 uint32_t calchash(const char *s);
+char const *print(char c);
 size_t readUTF8Char(uint8_t *dest, char const *src);
 
 #endif /* RGBDS_UTIL_H */