Also warn about non-used mid-rule values.

Also warn about non-used mid-rule values.
* src/symlist.h, src/symlist.c (symbol_list): Add a mid_rule
member.
(symbol_list_new): Adjust.
* src/reader.c (symbol_typed_p): New.
(grammar_rule_check): Use it.
(grammar_midrule_action): Bind a mid-rule LHS to its rule.
Check its rule.
* tests/input.at (AT_CHECK_UNUSED_VALUES): New.
Use it.
* tests/actions.at (Exotic Dollars): Adjust.