Change pe parser not to load full file data into memory

[joachimmetz]

Currently the pe parser loads the full file data into memory, due to the PE class interface. Discussed this with the author of pefile and he indicates that wrapping the file-like object into a data slice could do the trick without having to change pefile.

• add data slice helper to dfvfs Added data slice helper dfvfs#403
• add len function to data slice helper Added len function to data slice helper dfvfs#405
• create a new dfvfs release
• change pe parser to use data slice Changed PE parser to use dfVFS data slice #2714 #2715