Meta: more modular pieces

[seanmonstar]

This is meant as a meta tracking issue for making reqwest more modular. It's actually in a project: modular reqwest

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)

[HaoliangXu]

Just a question, do we refactor the existing reqwest or create a new repo for the modular one?

[seanmonstar]

This is meant to all happen within the reqwest repo. Most of what is described can be done by refactoring internals, and doesn't need to be breaking changes.

[Wodann]

Would be great to get this in: #557

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

Even being able to pass in your own tokio::Runtime would be valuable, as this is required when sharing a single tokio::Runtime between multiple shared libraries - since each shared library expects its own globally initialised tokio::Runtime in reqwest now.