Rename TypeMap to Extensions

[yoshuawuyts]

Implements #116. Thanks!

Screenshots

[Fishrock123]

As a newcomer, this seems more clear.

Edit: although, this seems a lot like "continuation local storage", and if it is what I think it is, users may be more familiar with that line of naming? e.g. LocalStorage?

[yoshuawuyts]

@Fishrock123 the issue I see with the word "local" is that the question becomes: "local to what?" LocalStorage may be scoped locally to the thread, runtime, framework, request, endpoint, or middleware. Extensions is less overloaded -- at worst it may not be clear what it extends, but the assumption will always be that it extends something in the crate -- and good docs can help explain what it does.

[Fishrock123]

What about ContinuationStorage then?

[yoshuawuyts]

@Fishrock123 if, like me, you don't know what a "continuation" is in this context, that name doesn't seem very intuitive. It's also fairly long. "Extensions" is shortened in the codebase as Request::ext() and Response::ext(). In contrast smth like Request::cont_store seems less intuitive to me, but also longer.

I feel overall "extensions" is the best fit we have. With the added benefit that other crates in the Rust ecosystem use the same name for the same concept as well.

[Fishrock123]

A bit late, but async_std does also use the term "local storage" for tasks, and the api is "LocalKey": https://docs.rs/async-std/1.5.0/async_std/task/index.html#task-local-storage