shada,functests: Test compatibility support

For compatibility the following things are done:

1. Items with type greater then greatest type are ignored when reading and
   copied when writing.
2. Registers with unknown name are ignored when reading and blindly copied when
   writing.
3. Registers with unknown type are ignored when reading and merged as usual when
   writing.
4. Local and global marks with unknown names are ignored when reading. When
   writing global marks are blindly copied and local marks are also blindly
   copied, but only if file they are attached to fits in the `'N` limit defined
   in &shada. Unknown local mark’s timestamp is also taken into account when
   calculating which files exactly should fit into this limit.
5. History items with unknown type are ignored when reading and blindly copied
   when writing.
6. Unknown keys found in register, local marks, global marks, changes, jumps and
   search pattern entries are read to additional_data Dictionary and dumped (of
   course, unless any of these elements were not overwritten later). It
   obviously works only for values conversible to Object type.
7. Additional elements found in replacement string and history entries are read
   to additional_elements Array and dumped (same: only if they were not
   overwritten later). Again this works only for elements conversible to Object
   type.
8. Additional elements found in variable entries are simply ignored when
   reading. When writing *new* variables they will be preserved during merging,
   but that’s all. Variable values dumped from current NeoVim session never have
   additional elements.

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