Remove intrusive_list (NFC)

[keithw]

This PR replaces the intrusive_list class with a std::list of unique_ptr's, trying to keep everything else mostly the same.

[sbc100]

Do you know the rationale for creating this class in the first place? Does the reasoning for it no longer hold?

[keithw]

It's not super clear (#542), but generally the reason to prefer an intrusive list vs. std::list is about performance (an intrusive_list only needs one allocation to make a list node with both the contained object and the prev/next pointers). And indeed, I just benchmarked this PR (with wasm-validate clang.wasm) and std::list is a significant slowdown for large modules. :-(

main branch:

time ./bin/wasm-validate ~/w2c2/examples/clang/clang.wasm

real	0m1.887s
user	0m1.451s
sys	0m0.436s

this PR:

time ./bin/wasm-validate ~/w2c2/examples/clang/clang.wasm

real	0m2.967s
user	0m2.531s
sys	0m0.436s

I wasn't expecting it to be this bad -- let's hold off on this for now until I find a better way.

[binji]

I believe I originally switched from a vector to an intrusive list, since I thought we'd want to be able to do insertions into the list. I'm not sure that was a good decision, tbh. It may make sense to see whether switching it back to a vector would work.

[keithw]

Well, here's a version with std::vector for the ExprList and std::deque for the ModuleFieldList (since we sometimes iterate over the ModuleFieldList while also appending to it...). Still unfortunately a performance regression, but not as bad as before:

$ time ./bin/wasm-validate ~/w2c2/examples/clang/clang.wasm 

real	0m2.456s
user	0m1.972s
sys	0m0.484s

[keithw]

Reverting to draft -- I think changing ModuleFieldList away from intrusive_list in a way that doesn't introduce a performance regression will take a more involved change. The Module structure has internal pointers pointing inside the ModuleFieldList, and likes to iterate over the ModuleFieldList while appending to it, so a naive replacement with vector isn't safe (and even deque fails -- in the fuzzer only, not in any of the asan spec tests, which spooks me a bit).