Some improvements to the cURL interface

[thomasp85]

This PR encompasses a couple of (in my opinion) improvements to how HTTP requests are handled.

Currently it makes the following changes:

• data can now be a raw vector instead of a string to avoid translating back and forth if the body is already in raw format
• The promise interface has been changed to return all relevant information from the response, instead of only the body. The response is now resolved to a list with the elements status, headers, and body. It no longer throws an error on non-2XX responses

I would love to have all headers returned by default (that is response = NULL returns all headers), but AFAIK NNG doesn't allow that for non-obvious reasons

This PR is mainly intended as an inspiration

[shikokuchuo]

@thomasp85 I'm closing this PR as your last remaining point about returning all response headers will now be implemented as part of #240 (specifically in commit d6b1718). This will be enabled by specifying ncurl(response = TRUE).

I implemented this for existing NNG without requiring the new 2.0 alpha builds by creating our own copies of the internal struct definitions, and defining our own iterable accessor.