api.value.type: implement proper support, check, and document

api.value.type: implement proper support, check, and document

* data/c.m4 (b4_symbol_type_register, b4_type_define_tag)
(b4_symbol_value_union, b4_value_type_setup_union)
(b4_value_type_setup_variant, b4_value_type_setup):
New.
(b4_value_type_define): Use it to set up properly the type.
Handle the various possible values of api.value.type.
* data/c++.m4 (b4_value_type_declare): Likewise.
* data/lalr1.cc (b4_value_type_setup_variant): Redefine.

* tests/types.at: New.
Exercise all the C/C++ skeletons with different types of
api.value.type values.
* tests/local.mk, tests/testsuite.at: Use it.

* doc/bison.texi (%define Summary): Document api.value.type.
* NEWS: Advertise it, together with api.token.constructor.