Enable experimental report features?

[Akim Demaille]

There is an item of TODO about which I'd appreciate the input of
others.  It is included below (it might help people who don't know the
gory details of LR that I explain `core item set': when there is an
item, say `$axiom -> . input $', where the current point is before a
nonterminal, then you have to add all the rhs of the rules deriving
`input' (transitivily).  For sake of memory saving, bison stores (and
displays) only the core items, but when computing the automaton, it
completes the core item set with the others, which are easily deducible):

* Experimental report features
Decide whether they should be enabled, or optional.  For instance, on:

        input:
          exp
        | input exp
        ;

        exp:
          token1 "1"
        | token2 "2"
        | token3 "3"
        ;

        token1: token;
        token2: token;
        token3: token;

the traditional Bison reports:

        state 0

            $axiom  ->  . input $   (rule 0)

            token       shift, and go to state 1

            input       go to state 2
            exp         go to state 3
            token1      go to state 4
            token2      go to state 5
            token3      go to state 6

        state 1

            token1  ->  token .   (rule 6)
            token2  ->  token .   (rule 7)
            token3  ->  token .   (rule 8)

            "2"         reduce using rule 7 (token2)
            "3"         reduce using rule 8 (token3)
            $default    reduce using rule 6 (token1)

while with --trace, i.e., when enabling both the display of non-core
item sets and the display of lookaheads, Bison now displays:

        state 0

            $axiom  ->  . input $   (rule 0)
            input  ->  . exp   (rule 1)
            input  ->  . input exp   (rule 2)
            exp  ->  . token1 "1"   (rule 3)
            exp  ->  . token2 "2"   (rule 4)
            exp  ->  . token3 "3"   (rule 5)
            token1  ->  . token   (rule 6)
            token2  ->  . token   (rule 7)
            token3  ->  . token   (rule 8)

            token       shift, and go to state 1

            input       go to state 2
            exp         go to state 3
            token1      go to state 4
            token2      go to state 5
            token3      go to state 6

        state 1

            token1  ->  token .  ["1"]   (rule 6)
            token2  ->  token .  ["2"]   (rule 7)
            token3  ->  token .  ["3"]   (rule 8)

            "2"         reduce using rule 7 (token2)
            "3"         reduce using rule 8 (token3)
            $default    reduce using rule 6 (token1)

so decide whether this should be an option, or always enabled.  I'm in
favor of making it the default, but maybe we should tune the output to
distinguish core item sets from non core:

        state 0
            Core:
            $axiom  ->  . input $   (rule 0)

            Derived:
            input  ->  . exp   (rule 1)
            input  ->  . input exp   (rule 2)
            exp  ->  . token1 "1"   (rule 3)
            exp  ->  . token2 "2"   (rule 4)
            exp  ->  . token3 "3"   (rule 5)
            token1  ->  . token   (rule 6)
            token2  ->  . token   (rule 7)
            token3  ->  . token   (rule 8)

            token       shift, and go to state 1

            input       go to state 2
            exp         go to state 3
            token1      go to state 4
            token2      go to state 5
            token3      go to state 6