[PSR-7] Rename StreamInterface to StreamableInterface

@auroraeosrose raised the point that the name `StreamInterface` connotes that
the object can be consumed as a PHP stream (i.e., that it acts as a stream
wrapper). Since it does not, renaming it to `StreamableInterface` makes sense
as it indicates the contents are something that can be streamed, but that the
implementation may not be an actual PHP stream.