Support multipart form data

[tmattio]

This adds support to decode multipart/form-data-encoded requests.

The parsing is delegated to https://github.com/cryptosense/multipart-form-data, so nothing too fancy here.

I added an example of a file upload using the provided functions. The example comes in two variants:

• examples/file_upload/main.exe that provides a nice UX and uses TailwindCSS and AlpineJS
  

• examples/file_upload/simple.exe that is much simpler and only uses what's necessary to implement the file upload
  

It would be nice to have support for file uploads directly in Opium, but I think it should live in Opium_kernel and we don't have a solution to implement IO functions at the moment. At the same time, passing an IO module doesn't make for a very nice API, so in the meantime, people can just copy-paste the example.

[rgrinberg]

Looks good. However, Lwt_stream is gross and we should avoid/discourage its use. Is it possible to use multipart without it?

[tmattio]

I initially wanted to use https://github.com/dinosaure/multipart_form, which does not use Lwt_stream, but it's not released on Opam. I'd be happy to use it instead of multipart-form-data if @dinosaure releases it on Opam at some point though.

But I'm curious: what's wrong with Lwt_stream that we should avoid using it?

[dinosaure]

@tmattio I can do a release of it if you want 👍

[tmattio]

@dinosaure That would be awesome! I'll update the code to use your implementation when it's on Opam :)

[rgrinberg]

what's wrong with Lwt_stream that we should avoid using it?

It has poor semantics