This commit allows blobs to be constructed from implementatons of the
'RawIOBase' ABC. This allows any conformant stream implementation to be added
as a blob.
This is useful in the case where the contents of the blob would be too large to
create from memory (as 'create_blob_fromstring' allows) and avoids having to
write the data to disk (as 'create_blob_fromworkdir' allows). The latter
operation is especially useful when reading data from a network socket, since
it avoids having to first commit all the data to disk.
9cce003efe