c++: revamp the support for variants

The current approach was too adhoc: the symbols were not sufficiently
self-contained, in particular wrt memory management.  The "new"
guideline is the one that should have been followed from the start:
let the symbols handle themslves, instead of leaving their users to
it.  It was justified by the will to avoid gratuitious moves and
copies, but the current approach does not seem to be slower, yet it
will probably be simpler to adjust to support move semantics from
C++11.

The documentation says that the %parse-param are available from the
%destructor.  In retrospect, that was a silly design decision, which
we can break for variants, as its a new feature.  It should be phased
out for non-variants too.

* data/variant.hh: A variant never knows if it stores something or
not, it is up to its users to store this information.
Yet, in parse.assert mode, make sure the empty/filled variants
are properly used.
(b4_symbol_constructor_define_): Don't call directly the symbol
constructor, to save a useless temporary.
* data/stack.hh (push): Steal the pushed value instead of duplicating
it.
This will simplify the callers of push, who handled this "move"
approach themselves.
* data/c++.m4 (basic_symbol): Let -1, as kind, denote the fact that
a symbol is empty.
This is needed for instance when shifting the lookahead: yyla
is given as argument to "push", and its value is then moved on
the stack.  But then yyla must be declared "empty" so that its
destructor won't be called.
(basic_symbol::move): New.
Move the responsibility of calling the destructor from yy_destroy
to ~basic_symbol in the case of variants.
* data/lalr1.cc (stack_symbol_type): Now a derived class from its
previous value, so that we can add a constructor from a symbol_type.
(by_state): State -1 means empty.
(yypush_): Factor, by calling one overload from the other one, and
using the new semantics of stack::push.
No longer reclaim by hand the memory from rhs symbols, since now
that we store objects with proper destructors, they will be reclaimed
automatically.
Conversely, be sure to delete yylhs.
* tests/c++.at (C++ Variant-based Symbols): New "unit" test for
symbols.

data/stack.hh

@@ -53,11 +53,15 @@ m4_define([b4_stack_define],
       return seq_[seq_.size () - 1 - i];
     }
 
+    /// Steal the contents of \a t.
+    ///
+    /// Close to move-semantics.
     inline
     void
-    push (const T& t)
+    push (T& t)
     {
-      seq_.push_back (t);
+      seq_.push_back (T());
+      operator[](0).move (t);
     }
 
     inline