Some functions from upstream VIM were reintegrated for this:
- get_option_value_strict
- set_option_value_err
- set_option_value_for
- unset_global_local_option
- Split functions with multiple files in the 'api' subdirectory
- Move/Add more types in the 'api/defs.h' header
- Add more prototypes
- Refactor scripts/msgpack-gen.lua
- Move msgpack modules to 'os' subdirectory