PR proposes applying source maps to wasm binaries

[okay49]

This PR proposes applying source maps to wasm binaries. Source maps obviously have a lot of known limitations and are not a long-term debugging solution. However they are widely supported (including by emscripten for asm.js code) and based on conversations with Chrome and Firefox dev tools folks, my hope is that it will be easy and straightforward to adapt the existing dev tools code to apply to wasm binaries in the meantime, and bring wasm to parity with asm.js.

[kripken]

This has already been implemented in some toolchains and browsers. Here is the spec:

https://github.com/WebAssembly/tool-conventions/blob/4d4bbcc74e36b646a9abda55cb4142c2fe848414/Debugging.md#source-maps

For an example of it, you can build something like this with emcc:

emcc tests/hello_world.c -gsource-map -o a.html

Then open a.html in a browser. For example in the Chrome devtools you can find tests/hello_world.c in the 'Sources' tab. I verified this works now in emcc 2.0.21 and Chrome 91.

(With all that said, though, DWARF debugging is usable today and offers many benefits: https://developer.chrome.com/blog/wasm-debugging-2020/)