Create a more ergonomic way to handle partial requests

[thomastaylor312]

The way things work right now (as seen in #171) is that when doing partial requests, the burden is on the user to validate the SHA of the data when complete. What we should create is a new PartialRequestStream (or a better name than that) type that impls Stream and can be constructed from an offset. Honestly this could even just replace the current return types from the blob stream methods. It would need to track current progress if a partial_content response is received and update an internal Digester with all of the data (similar to what VerifyingStream does now) that could be validated when complete. A single response type like this would make things more ergonomic for users and make sure we can validate everything