Make read_bytes() also return the number of bytes read

[vmx]

If you traverse a file containing CIDs, you might want to keep track of your current position. Currently read_bytes() only returns the new CID. As varints are involved, you cannot tell by how many bytes the reader was advanced. Hence I propose extending read_bytes() to also return the number of byte read.

[mriise]

this would require api changes in both varint and multihash, and is a bit awkward as bytes read would have to be passed up in both cases somehow.

is there a specific usecase where this would be useful?

[vmx]

I was building an index on top of a file that stores CID + data tuples. I wanted to get the byte offset of the data without asking the reader about its current position after it read a CID.