Rewrite tracing middleware

[jplatte]

There are a few problems with the current tracing middleware:

• It is overly generic, making it hard to grok
• The body type it uses doesn't (ever) implement Default, but other middleware requires this bound

It would be nice if we could come up with a different tracing middleware that fulfills most or all of the current ones' use cases without sharing these problems.

[Oliboy50]

The body type it uses doesn't (ever) implement Default, but other middleware requires this bound

Can you elaborate on how it impacts users, please?
Because we use this layer along with other ones from this repository and have no issue so far.

[jplatte]

It leads to some chains of middlewares not compiling. If you've got a project that builds successfully, it doesn't affect you.

[jplatte]

I've taken a first step in #380, which only simplifies the implementation, not the interface.

Having looked at most of the code though, one piece in particular that stands out as really complex / abstract is the classify API. I have never used it and wonder if it's worth keeping it at all. @davidpdrsn how much of this are you using, and are you aware of others using it?