Add Request and Response blob methods

[andreiltd]

This patch adds blob API to Request and Response methods.

The patch also refactors the internal blob storage, changing it from std::vector to mozilla::Vector. This optimization allows the Blob to be created without copying data by transferring buffer ownership, as is the case for both Response and Request objects.

Closes: #121

[tschneidereit]

So many more passing tests! ❤️

[guybedford]

It would also be nice to see in this PR for return new Response(blob) setting the content-type header from the blob type (see the Blob path in https://fetch.spec.whatwg.org/#concept-bodyinit-extract).

Update: I was able to implement these for Fastly here - fastly/js-compute-runtime@9b59afa

[guybedford]

I've also posted #182 as a further follow-up.

[andreiltd]

Update: I was able to implement these for Fastly here - fastly/js-compute-runtime@9b59afa

Nice! This looks to me like an equivalent of doing:

RootedValue blob_stream(cx);
Call(cx, body_obj, "stream", HandleValueArray::empty(), &blob_stream);

Is there a reason we don't want to use Call? Is this indirection not performant enough? If that's the case I should probably add API that allows calling blob methods like stream, bytes etc. directly. WDYT?

[andreiltd]

I've also added support for Blob in structured-clone but perhaps this should be in another PR. Let me know if you want me to split this.

[guybedford]

@andreiltd yeah in general I'd avoid running through the high-level call for performance - we very much need to always keep performance top of mind in development on StarlingMonkey, because by default we are running at a performance penalty compared to other JS platforms, performance is of critical performance for this codebase.

[andreiltd]

That makes sense. I will make sure to add interface for calling Blob functions directly in the File API PR.