Automatically handle http_body::LengthLimitError

[davidpdrsn]

This adds special handling of http_body::LengthLimitError to FailedToBufferBody such that this just works and returns 413 Payload Too Large if the limit is exceeded.

tower-http's new RequestBodyLimitLayer uses that so limiting both requests with a known length and streaming is just:

use tower_http::limit::RequestBodyLimitLayer;

Router::new()
    .route("/", post(|body: Bytes| { ... }))
    .layer(RequestBodyLimitLayer::new(1024));

---

I think changing ContentLengthLimit to use http_body::Limited would be good but we cannot do that without breaking changes since it requires T: FromRequest<B> but we have to change that to T: FromRequest<Limited<B>>.

EDIT: Actually I don't think we can do that since you cannot change the body type in a RequestParts with only an &mut RequestParts. You'd need an owned value 🤔