RFC: My implementation of a body for Request

[StratusFearMe21]

I went ahead and made the PR of the API I mentioned in #3, So far it works great and is very performant, but I'd still like to know what y'all think about my implementation.

[StratusFearMe21]

I've just added this comment on line 52 of request.rs under the Body's resolve() function

/// This is preferable over using `std::io::Read` if your `Body` is small.

I'm pretty sure this is correct, but I'm not 100% sure.

[StratusFearMe21]

I've just improved the safety of Request using lifetime magic, however, it is a breaking change (but considering this project is unreleased that probably doesn't matter)

[Xudong-Huang]

I like the impl, but could we first make a feature guard to avoid affect the existing project?

[Xudong-Huang]

we can first merge the this into dev branch, we would merge it back to mater when matured

[RoadtogreatWorld]

Can this already be merged into master? I need to use it and dev branch is so behind.