README: update and simplify the introduction
sl/symutil: adopt canPointTo{Front,Back} from symabstract

... originally named filter{Front,Back}
sl/shape.hh: common shape definition for cont_shape and symdiscover
sl/symdiscover: hide the implementation of isDlsBinding()
sl/symdiscover: unimplement isDlsBinding() completely
sl/symdiscover: export the canMergeObjWithNextObj() function