Add support for non-async writing

[rambip]

Hi !
I need a library to write CAR files in rust, and this crate looks really promising.

The issue is that I need to run this code client-side (and in the future on webassembly), so an async implementation is a dealbreaker for me.

Would that be possible to export a non-async writer ?

[dignifiedquire]

You can use async code in webassembly as well, using the futures executor. Could you expand on the setup you need to run in, where you can't use async?.

[rambip]

I'm building a static-site generator, (as a native rust app) and the idea is to publish some content to ipfs, getting the CIDs, and the building the website.
I use the HTTP api of <web3.storage> to upload the car (with reqwest )
So there is really no need for async for me.

[dignifiedquire]

So there is really no need for async for me.

sure, but the sync api in reqwest just blocks under the hood