Simpler custom requests

[pcantrell]

This fixes #152, adding a simpler, better way for apps to create custom Siesta Requests that are completely detached from the underlying network provider. Such requests can for example:

• wrap a third-party library’s network request (e.g. OAuth),
• wrap a local load operation (e.g. deserialization) as a request, or
• implement a time delay

…and when these become Requests, they can all participate in a request chain. This allows, for example, a chain like “on failure, renew the OAuth token, delay 3 seconds, then retry.”

To use the new API, apps implement RequestDelegate and pass it to Resource.prepareRequest(using:). See details in the API docs in the PR.

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This PR also adds Resource.hardWiredRequest(returning:), for situations when you already have a fixed response, but want to wrap it as a request. This is a generalization of the existing Resource.failedRequest(…).