Create basic file archive code

The password vault will be an archive format similar to tar. The specific requirements are:

• The archive must store any number of named items
• Items must be able to be opened and written/read by name
• The individual item content must be stored in contiguous sections (future revisions may change this).
  • This will simplify the encryption decryption code that will use this (libsodium secret_stream).
  • This will simplify file writing code.
• It should be possible to seek to specific items without having to first read the entire archive (i.e. it should have an index in a known location).
• Items names shall be opaque byte sequences and shall not be interpreted as ascii or utf-8 (future revision may change this).