0.18.7: Add merge() fusion

• Add fusion for merge() and mergeArray()
  • Nested merge structures, like merge(a, merge(b, c)), a.merge(b).merge(c), or mergeArray([a, merge(b, c), d.merge(e)] are flattened into a single node in the stream graph. There's now zero performance difference in a.merge(b).merge(c).merge(d) and mergeArray([a, b, c, d])