Define the constructors of symbol_type in b4_symbol_constructor_definitions.

Define the constructors of symbol_type in b4_symbol_constructor_definitions.

The constructors are called by the make_symbol functions, which a
forthcoming patch will move elsewhere.  Hence the interest of putting them
together.

The stack_symbol_type does not need to be moved, it is used only by the
parser.

	* data/lalr1.cc: Move symbol_type and symbol_base_type
	constructors into...
	(b4_symbol_constructor_definitions): here.
	Adjust.