Implement 0-len/drop spec changes in bulk memory (#2529)

This implements recent bulk memory spec changes
(WebAssembly/bulk-memory-operations#126) in Binaryen. Now `data.drop` is
equivalent to shrinking a segment size to 0, and dropping already
dropped segments or active segments (which are thought to be dropped in
the beginning) is treated as a no-op. And all bounds checking is
performed in advance, so partial copying/filling/initializing does not
occur.

I tried to implement `visitDataDrop` in the interpreter as
`segment.data.clear();`, which is exactly what the revised spec says. I
didn't end up doing that because this also deletes all contents from
active segments, and there are cases we shouldn't do that:
- `wasm-ctor-eval` shouldn't delete active segments, because it will
  store the changed contents back into segments
- When `--fuzz-exec` is given to `wasm-opt`, it runs the module and
  compare the execution call results before and after transformations.
  But if running a module will nullify all active segments, applying
  any transformation to the module or re-running it does not make any
  sense.

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