feat: slicing a Blob

[grabbou]

Previous implementation of Blob was naive, because it sliced everything in memory, assuming contents were already loaded. In reality, Blob arrayBuffer() or bytes() are async and lazy. However, slice is synchronous.

We should implement something similar to a BlobView that gets created as a subclass of a Blob, holds information about offset/length of a slice and will still ready full underlying stream(), but return its slice.

Here, it might be relevant to implement seeking a stream (as discussed with @mani3xis)