Factor out the eml templater?

[aantron]

@tmattio, could you ping this issue when you are either soon to get started using eml outside Dream (it's very quick to factor out), or decide not to?

[tmattio]

I'm already using it in an upcoming PR to https://github.com/ocaml-doc/voodoo. Nothing is blocked at the moment, I just added a dependency on Dream directly, but I'll switch to eml if/when it is released independently. I can ping you once the PR lands in the repo above if you prefer to wait.

cc @jonludlam

[aantron]

Please ping me when you definitely need it, if it's not done yet. It's not that I'm waiting, rather choosing what to work on next, so if I don't get it done by when you need it (i.e. so your PR doesn't depend on like ~100+ packages that Dream transitively depends on :P), let me know so I can prioritize it :) eml itself IIRC has no deps (or almost none).

[aantron]

One of my concerns was that I had a slightly more "flexible" design in mind for an "independent" eml, one where the user/integrator had more control over what eml actually emits (the code that makes strings, writes to buffers or streams, etc.), but I didn't finish designing it because there was no need yet. Let me know if you have any input on that sort of thing, based on your work.