Create a safe, straightforward API to capture JS stacks

[jdm]

https://github.com/servo/mozjs/blob/3f38435122538e4b66f88878302f81331c0b6a44/mozjs/js/src/jsapi.h#L5895-L6039 looks like it gives us the building blocks we need in C++. We should make it as easy to capture JS stacks as possible and use them to help us debug problems with web compatibility.

[jdm]

The idea here is that CaptureCurrentStack gives us a JSObject that represents a captured stack. BuildStackString accepts a JSObject representing a captured stack and yields a string representation of the stack. A great way to start would be to make a Rust type that captures the stack in its constructor, and has a method that returns a Rust string representation. This type will need to refer to store a RootedGuard, meaning that it refers to a rooted JS object that was created earlier in the program. This object will store the captured stack.

[jdm]

It would make the most sense to build this in the rust-mozjs repository, rather than in servo itself.

[mrowqa]

Hi, I will work on this. I have already read the code around and this seems like quite easy task.

[jdm]

@mrowqa Great! Please ask questions if anything is unclear!

[mrowqa]

@jdm Is that all or do we want something more? And thank you for the help :)

[jdm]

Good enough for now!