Meta: more modular pieces

[seanmonstar]

This is meant as a meta tracking issue for making reqwest more modular. It's actually in a project: https://github.com/users/seanmonstar/projects/3

But this being an issue means it can be pinned in the reqwest issue tracker.

For more background information, see this announcement: https://seanmonstar.com/blog/modular-reqwest/

[flying-sheep]

Would be great to get this in: #557

Together with a demo that uses reqwest without tokio (but a different runtime instead)