docs(lib): fix several broken links throughout docs

Closes #1749
Closes #1750

src/body/mod.rs

@@ -7,7 +7,7 @@
 //!
 //! There are two pieces to this in hyper:
 //!
-//! - The [`Payload`](Payload) trait the describes all possible bodies. hyper
+//! - The [`Payload`](body::Payload) trait the describes all possible bodies. hyper
 //!   allows any body type that implements `Payload`, allowing applications to
 //!   have fine-grained control over their streaming.
 //! - The [`Body`](Body) concrete type, which is an implementation of `Payload`,

src/body/payload.rs

@@ -6,7 +6,7 @@ use super::internal::{FullDataArg, FullDataRet};
 
 /// This trait represents a streaming body of a `Request` or `Response`.
 ///
-/// The built-in implementation of this trait is [`Body`](Body), in case you
+/// The built-in implementation of this trait is [`Body`](::Body), in case you
 /// don't need to customize a send stream for your own application.
 pub trait Payload: Send + 'static {
     /// A buffer of bytes representing a single chunk of a body.