* src/reader.c (copy_at, copy_dollarm parse_braces, parse_action)

* src/reader.c (copy_at, copy_dollarm parse_braces, parse_action)
(parse_guard): Rename the formal argument `stack_offset' as
`rule_length', which is more readable.
Adjust callers.
(copy_at, copy_dollar): Instead of outputting the hard coded
values of $$, $n and so forth, output invocation to b4_lhs_value,
b4_lhs_location, b4_rhs_value, and b4_rhs_location.
* data/bison.simple, data/bison.c++ (b4_lhs_value)
(b4_lhs_location, b4_rhs_value, and b4_rhs_location: New.