Add "interposition" to the fuzzer's mutate() method (#6427)

Before this PR we only mutated by replacing an expression with another, which
replaced all the children. With this PR we also do these two patterns:

(A
  (B)
  (C)
)

=> ;; keep children, replace A

(block
  (drop (B))
  (drop (C))
  (NEW)
)

,

(D
  (A
    (B)
    (C)
  )
)

=> ;; keep A, replace it in the parent

(D
  (block
    (drop
      (A
        (B)
        (C)
      )
    )
    (NEW)
  )
)

We also try to replace onto the new D (either A itself, or A's children).

0 files changed, 0 insertions, 0 deletions