Unify operator_cst range operator

Move the declaration of the class to the range-op-mixed header, add the
floating point prototypes as well, and use it in the new unified table.

	* range-op-float.cc (operator_cst::fold_range): New.
	* range-op-mixed.h (class operator_cst): Move from integer file.
	* range-op.cc (op_cst): New object.
	(unified_table::unified_table): Add op_cst. Also use for REAL_CST.
	(class operator_cst): Move to range-op-mixed.h.
	(integral_table::integral_table): Remove op_cst.
	(pointer_table::pointer_table): Remove op_cst.