variant: more assertions

variant: more assertions

Equip variants with more checking code.  Provide a means to request
includes.

* data/variant.hh (b4_variant_includes): New.
* data/lalr1.cc: Use it.
* data/variant.hh (variant::built): Define at the end, as a private member.
(variant::tname): New.
Somewhat makes "built" useless, but let's keep both for a start, in
case using "typeinfo" is considered unacceptable in some environments.
Fix some formatting issues.