Split sync and async body into distinct types

[sagebind]

Make the big split between synchronous responses and asynchronous responses, which offers ergonomic improvements and encourages users towards using the right methods by default as discussed in #202. This required the following changes:

• Split Body into two distinct types: AsyncBody and Body, which are asynchronous only and synchronous only, respectively.
• Split ResponseExt into three traits: ResponseExt for methods that don't use the body, ReadResponseExt for synchronous methods that read from the body, and AsyncReadResponseExt for async methods that read from the body. This offers some ergonomic improvements as well, as the async response methods can drop the _async suffix, and Rust will automatically resolve to the correct method depending on whether the response is async or not.

This split also enables a new capability that I did not think of before, which is that you can now use anything implementing Read as a request body such as File when using the synchronous API. For those not using async at all, this is a significant improvement!

Fixes #202.

[codecov]

Codecov Report

Merging #262 (936970e) into master (6a399a2) will increase coverage by 0.69%.
The diff coverage is 83.02%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   71.84%   72.53%   +0.69%     
==========================================
  Files          49       51       +2     
  Lines        2515     2713     +198     
==========================================
+ Hits         1807     1968     +161     
- Misses        708      745      +37     

Impacted Files	Coverage Δ
src/default_headers.rs	100.00% <ø> (ø)
src/interceptor/mod.rs	0.00% <0.00%> (ø)
src/redirect.rs	76.47% <ø> (ø)
src/request.rs	27.45% <ø> (ø)
tests/redirects.rs	100.00% <ø> (ø)
src/headers.rs	47.05% <47.05%> (-52.95%)	⬇️
src/response.rs	60.86% <66.66%> (-21.49%)	⬇️
src/client.rs	65.63% <69.13%> (-4.03%)	⬇️
src/body/mod.rs	81.30% <81.30%> (ø)
src/body/sync.rs	81.81% <81.81%> (ø)
... and 15 more

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