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).

src/nvim/shada.h

@@ -0,0 +1,18 @@
+#ifndef NVIM_SHADA_H
+#define NVIM_SHADA_H
+
+typedef long ShadaPosition;
+
+/// Flags for shada_read_file and children
+enum {
+  kShaDaWantInfo = 1,     ///< Load non-mark information
+  kShaDaWantMarks = 2,    ///< Load file marks
+  kShaDaForceit = 4,      ///< Overwrite info already read
+  kShaDaGetOldfiles = 8,  ///< Load v:oldfiles.
+};
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "shada.h.generated.h"
+#endif
+
+#endif  // NVIM_SHADA_H