read: remove r5js.DatumType.SPLICING_UNQUOTATION by adding a r5js.Datum subclass.
read: remove r5js.DatumType.UNQUOTE by adding a r5js.Datum subclass.
read: remove r5js.DatumType.QUASIQUOTE by adding a r5js.Datum subclass.
misc: make processQuasiquote a method on r5js.Quasiquote.
datum: simplify decorateQuasiquote methods.
datum: remove r5js.Datum#isQuasiquote in favor of direct instanceof checks.
datum: remove r5j.Datum#isUnquote in favor of direct instanceof checks.
datum: rename r5js.Datum#decorateQuasiquote => #setQuasiquotationLevel.