dyn HttpClient

[Fishrock123]

This is a rebase of @islandusurper and @icewind1991's commits as linked in https://github.com/http-rs/surf/issues/69, based on an http-client branch which actually fixes the Sized and Error bounds issues on the HttpClient trait.

This makes Client no longer a generic, simplifying things as noted in https://github.com/http-rs/surf/issues/192 and also allowing things such as https://github.com/http-rs/surf/issues/150 (static client), while also fixing other reported issues.

Todo:

• [x] Merge https://github.com/http-rs/http-client/pull/31
• [x] Merge https://github.com/http-rs/http-client/pull/30
• [x] Merge https://github.com/http-rs/http-client/pull/29
• [x] http-client release
• [x] rebase/squash (remove git dep + always compiling commits(?))