Resolve ambiguity of the term "blob reference"

[jorisdral]

We've previously used the term BlobRef for both (i) a handle-like reference to a blob in a blob file, and (ii) for the concrete offset to and size of a blob in a blob file. #81 introduces the BlobSpan type for (ii). We should ensure that we use BlobRef for (i) and BlobSpan for (ii) consistently throughout the code and documentation.