Switch from `use … as IpfsClient` to type aliases for clearer API docs.

[nathan-at-least]

I find the generated API docs for use path::Foo as Bar to be confusing compared to type aliases.

Current docs from master branch

Currently in master, the identifier ipfs_api_backend_hyper::IpfsClient is defined as:

pub use crate::{backend::HyperBackend as IpfsClient, error::Error};

The rendered API docs look like this:

What is the relationship of IpfsClient to HyperBackend?

I believe the root problem here is actually a rustdoc problem, but this PR uses type aliases to work around the ambiguity.

With type aliases (this PR) in HyperBackend

By introducing type aliases first of all the alias is documented explicitly:

-then when clicking through to the definition of HyperBackend the API is unambiguous:

With type aliases (this PR) in ActixBackend

The equivalent for ActixBackend has this type alias:

-and this underlying backend API:

[ferristseng]

Thanks! This looks great!

[iamjpotts]

@ferristseng did this change break the CI build?