Adding mutation metadata as a custom section

In `wasm-mutate`, what about adding the mutation history in a custom section ?

Each mutation (optionally) can be recorded as a new entry in the `wasm-mutate` custom section. The entry can be something like, the mutator name and the seed used over the input Wasm. This can be optional. It can help to identify binaries generated out of wasm-mutate. Besides, the mutation history can be used to better track a potential bug.

CC @fitzgen