ipfs-unixfs: no_std and alloc -ification for wasm use

[koivunej]

Creating this issue following interest expressed in the comments of #222. I don't have much to go on from my previous comment on this topic.

[nuke-web3]

I am finally getting a start on this, and my entry into this got a minor error trying to cargo build an example (#266) . Based on your last comment, I may be going down the wrong path?

[koivunej]

Would the use of bytes in ipfs-unixfs hurt this goal, or is it no_std compat or could we somehow be generic over the buffer type? It would of course be nice to seek out less allocation or less copying solutions as well.

[ljedrz]

As far as I can tell, bytes is no_std-aware and uses core and alloc internally. Enabling (default) std utilizes IoSlice.

[koivunej]

Well, that's just excellent. Yeah, need to do some bytes exploration, that might allow a path to much simpler impl and better perf. I'll try to write the thoughts up... Later.