Add the ability to parse types.

This ability is not integrated with the rest of the parser - in fact, the grammar for things like function declarations (where type declarations are necessary) is not yet determined.

include/lexer.h

@@ -62,6 +62,7 @@ enum libab_lexer_token_e {
     TOKEN_KW_ELSE,
     TOKEN_KW_WHILE,
     TOKEN_KW_DO,
+    TOKEN_KW_ARROW,
     TOKEN_LAST
 };