0.5: All the Async!

This is a huge release for cHTTP that delivers on first-class async/.await and big API ergonomic improvements! Read @sagebind's blog post if you want to learn even more about the release and the project's direction!

Breaking Changes

• Rename Client to HttpClient and ClientBuilder to HttpClientBuilder. For usability it helps to include the client type in our prelude, but the name Client is just a little too generic to do that. Using the name HttpClient is much less ambiguous as to what kind of client is in question. It's also a very common type name for HTTP clients and adds to familiarity. (#46)
• The Options struct has been removed. Instead, request execution options can be set by using extension methods provided for http::request::Builder, or by setting default configuration using methods available on HttpClientBuilder. This greatly improves the ergonomics of setting various options, as well as allows you to override specific options piecemeal instead of overriding everything. (#35)
• Body is now asynchronous internally. It still implements Read for ease of use, but you can no longer create a body from a synchronous reader. This fixes latency issues inside the agent thread, where previously sending and receiving bodies might block the entire event loop and prevent other parallel requests from making progress for a short time.
  This also means that you can optionally read from a response Body within an asynchronous application without blocking. (#27)
• The json feature no longer uses the json crate, and instead uses serde to automatically deserialize a JSON response to the target type.
• Rename the options module to config, and make several other redundant modules private.
• Removed Request and Response

Added

• Add a new chttp::prelude module that can be glob-imported to pull in the core types and traits you'll likely need to work with cHTTP.
• In addition to the existing synchronous send, get, post (etc) methods, asynchronous versions of these methods are now available, suffixed with _async. These methods do the exact same things as the synchronous versions, except entirely asynchronously and return a Future. (#27)
• When creating a request body from a reader, you can now provide a length hint if you know the size of the body ahead of time. When providing a length hint, the hint will be used as the value of the Content-Length request header and be sent directly, instead of using a streaming encoding.
• Add convenience methods copy_to and copy_to_file for downloading a response body to a file or an arbitrary writer.

Improvements

• The background agent thread that manages active requests has received many changes and improvements and is now fully asynchronous This offers less latency when multiple parallel requests are active and increased overall performance.
• Response buffers now use sluice which greatly increases the efficiency of reading from the HTTP response stream.
• Trim the dependency tree of crates that we don't really need, greatly improving both compile times and binary footprint. (#43) @sagebind
• Improve how we handle the Public Suffix List by bundling a copy via Git and refreshing it periodically during run time. This also removes the psl crate, which has been to compile very slowly. (#40)

Fixed

• Fix a bug where sending a HEAD request might hang waiting on the response until the server closes the connection.
• Fix various potential bugs in how certain HTTP verbs are handled by using explicit curl options for a verb if one exists.
• Fix some warning emitted by Clippy.

Changed

• Dropping a Client will now block until the associated agent thread shuts down. If the agent thread errors, it will propagate back to the main thread.
• Replace usages of Rouille with Mockito in integration tests, as it has a lot of nice convenience methods and is a little lighter on the dependencies.