Change merge strategy of descriptors, when using named descriptors

At this line the Object.assign is used to add properties of the named descriptor (if found) to the resulting one. This was OK when descriptors were objects with a flat property structure, but with the introduction of composite validator (see #15 (closed)) this doesn't work correctly: if both descriptors have composite validator, the one that's provided earlier is lost:

const compositeFirst = {
  value: {
    kind: "primitive-non-void",
    typeOf: "number",
  },
};

const compositeSecond = {
  value: {
    set: [ 2, 3 ],
  },
};

valuer.define(":digit", { composite: compositeFirst });
valuer.define(":two-or-three", { composite: compositeSecond });

valuer.as(":digit", ":two-or-three");
// (descriptor).composite === compositeSecond