[wasm-split] Slow splitting many modules

[biggs0125]

For a large file, wasm-split with --multi-split takes 3.5+ minutes to separate the file into ~350 submodules.

My understanding is that --multi-split basically invokes split mode iteratively. I imagine it would be faster if it processed the entire main module at once [O(n)] rather than walking the entire (albeit slowly shrinking) main module many times [O(n^2)].

[tlively]

cc @aheejin