Move streaming reader out of Blob into a dedicated BufReader class

[andreiltd]

This patch extracts the streaming reader logic from Blob and into a new BufReader class. This change removes A LOT of clutter from Blob while clearly separating the streaming concerns into a dedicated component.

The BufReader encapsulates the asynchronous task of reading data from a underlying source. It takes a Read function as a constructor parameter, allowing callers to specify how bytes are fetched into the provided buffer. The Read function must return the number of bytes read and indicate completion by setting done when there is no more data to read.

This refactoring is a preparatory work for a streaming FormDataEncoder, as discussed here: #191 (comment).

[tschneidereit]

This all makes sense to me, and it'll be great to be able to reuse things for streaming from FormData instances