Replace middleware API with interceptors API

[sagebind]

Implement an interceptors API and remove the old middleware API.

The new interceptors API is gated behind the unstable-interceptors feature flag. We sort of need to rush this out the door in order to implement #215 properly, but we're not ready to finalize the design yet. There are still some open problems to fix with this:

• from_fn doesn't work as desired. The trait bounds are too ambiguous for the compiler to infer for closures, and since the return type is generic over a lifetime, there's no way to give the return type the correct name using current Rust syntax.
• InterceptorObj wraps the returned future in an extra box.
• If an interceptor returns a custom error, it is stringified and wrapped in Error::Curl. We should introduce a new error variant that boxes the error and also records the type of the interceptor that created the error for visibility.
• Automatic redirect following currently bypasses interceptors for subsequent requests. This will be fixed when redirect handling is rewritten as an interceptor itself. See Cookies aren't sent when following redirects #232.

Users who are eager for interceptors can also opt-in to this feature and start experimenting if they can't wait for stabilization.

Related: #42

[codecov]

Codecov Report

Merging #206 into master will decrease coverage by 0.58%.
The diff coverage is 25.71%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   71.68%   71.09%   -0.59%     
==========================================
  Files          42       44       +2     
  Lines        2137     2128       -9     
==========================================
- Hits         1532     1513      -19     
- Misses        605      615      +10     

Impacted Files	Coverage Δ
src/cookies/mod.rs	83.60% <0.00%> (-1.40%)	⬇️
src/interceptor/mod.rs	0.00% <0.00%> (ø)
src/interceptor/obj.rs	0.00% <0.00%> (ø)
src/lib.rs	75.00% <ø> (ø)
src/interceptor/context.rs	30.00% <30.00%> (ø)
src/client.rs	66.35% <54.54%> (-2.27%)	⬇️

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