Add stacktrace API

[rossberg]

Provides API to get the frame from which a trap originates as well as an entire stacktrace. For each frame, instance, module offset, function index, and function offset can be inquired.

These functions are considered to be "best effort", i.e., an engine doesn't need to provide the information and can return null instead in some or all cases. The current V8 prototype always returns null for the origin, since the respective information isn't easily available through the V8 API.

See here for the interface.

If an embedder wants to associate file name information with an instance, then it can do so by setting the file name as host info on an instance object.

Addresses #54. @nomeata, @titzer

[nomeata]

I only see `module_offset , not function index or function offset, in the diff (but my ability to read C++ is limited).

[rossberg]

@nomeata, oops, forgot to push. PTAL.

[titzer]

API lgtm. This would fulfill my use case, and I think it's sufficiently general to allow other embedders to implement source-level stacktraces however they like.

If understand correctly, the "origin" is just the frame in which the trap actually occurred, not the entire stack trace. In the V8 implementation, this should be the topmost frame, if the trap is actually caused by Wasm (and not an exception thrown by calling out to the embedder).

[rossberg]

@titzer, yes, origin() is just an efficient shorthand for trace()[0], in case you only care about that. I could also imagine that some engines provide origin but not a full trace.

[titzer]

@rossberg Cool, that matches my understanding then. Still lgtm.

[titzer]

Btw, did you encounter a case where the origin frame is not correct? It should be in the full stack trace in V8, and if it is not, then this is a bug.

[rossberg]

Since I don't know how to actually get this data through the V8 API (esp. the instance), I haven't implemented it in the prototype -- origin always returns null atm. So no. :)

@jakobkummerow will hopefully know how this can be implemented internally.