parser: no longer use the "braceless" non-terminal

parser: no longer use the "braceless" non-terminal

The purpose of this symbol was only to factor function calls.  As a
result the actions were indeed simpler, but the grammar was somewhat
uselessly obfuscated.  Get rid of this symbol, but introduce functions
to simplify dependencies.

There is no (intended) changes of behavior here.

* src/parse-gram.y (strip_braces, translate_code(
(translate_code_braceless): New.
(braceless): Remove, use "{...}" instead, and one of the previous
functions depending on the context.
(STRING, "%{...%}", EPILOGUE): Declare as <code>, instead of <chars>,
the difference between both is useless (well, I couldn't make sense of
it, even after having read the initial commit that introduced them).
(%union): Remove the now useless "chars" type.
Adjust the printers.
* src/scan-gram.l: Adjust.