Correctly set the value of decode.bytes to bytes used.

[ckcr4lyf]

What is the purpose of this pull request? (put an "X" next to item)

[ ] Documentation update
[X] Bug fix
[ ] New feature
[ ] Other, please explain:

What changes did you make? (Give an overview)
After decoding is complete, update the value of decode.bytes to decode.position to reflect the number of bytes used

Which issue (if any) does this pull request address?
#178

Is there anything you'd like reviewers to focus on?
The original spec is here: https://github.com/mafintosh/abstract-encoding/ , the README for this project says it is "compatible", I would think this includes this property.

This might be slightly related / affect #61, since this strictly assumes we try and parse a single bencoded piece of data (e.g. two consecutive integers like i123ei123e are technically separate).

This PR becomes important when trying to bencode e.g. the response for a ut_metadata request (https://www.rasterbar.com/products/libtorrent/extension_protocol.html) , since the bencoded data is immediately followed by the binary metadata piece, so returning the number of bytes consumed enables offsetting into the binary data accurately.

A workaround I am using right now is to call encodingLength on the decoded data to determine how many bytes it would've been, but obviously this is wasteful.