What works:
1. ShaDa file dumping: header, registers, jump list, history, search patterns,
substitute strings, variables.
2. ShaDa file reading: registers, global marks, variables.
Most was not tested.
TODO:
1. Merging.
2. Reading history, local marks, jump and buffer lists.
3. Documentation update.
4. Converting some data from &encoding.
5. Safer variant of dumping viminfo (dump to temporary file then rename).
6. Removing old viminfo code (currently masked with `#if 0` in a ShaDa file for
reference).
