Allow (custom) sections to be more than 4GB in size?

[dschuff]

The spec for the module format limits the section size to be a u32, meaning sections can't be more than 4GB in size. But it's entirely possible for (for example) debug info to be larger than that.
For the LLVM ecosystem we will likely come up with a scheme that stores debug info separately from the loaded wasm binary, but it might be reasonable to use wasm-format containers to store this information (as we do for object files).

Should we allow sections (or maybe just custom sections) to be larger than expressible in u32? It would be a backwards-compatible change since use LEB everywhere; we'd just have to raise the limit in the tools. (Browsers might not want to allow such large sections anyway; we could put a limitation there alongside the other embedder-specific limits).

[binji]

Sgtm, next steps would be bringing to CG.