viminfo: First version of ShaDa file dumping

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).

contrib/YouCompleteMe/ycm_extra_conf.py

@@ -9,7 +9,7 @@ def DirectoryOfThisScript():
 
 def GetDatabase():
     compilation_database_folder = os.path.join(DirectoryOfThisScript(),
-                                               '..', 'build')
+                                               '..', '..', 'build')
     if os.path.exists(compilation_database_folder):
         return ycm_core.CompilationDatabase(compilation_database_folder)
     return None