API Fixes: Remove unnecessary NULL checks
API Fixes: Check that error isn't set in buffer_get_line
API Fixes: Remove wrong increment statement from buffer_set_slice
API Fixes: Implement api/handle module

This module will be used to implement remote management of objects through the
API. Object types to be registered must have a `uint64_t` field named 'handle'.
API Fixes: Register/unregister created/destroyed buffers

- Add the 'handle' field to `buf_T`
- Add declare/implement functions for registering/unregistering/retrieving
  buffers
- Register/unregister buffers when they are created/destroyed.
API Fixes: Register/unregister created/destroyed windows

- Add the 'handle' field to `win_T`
- Add declare/implement functions for registering/unregistering/retrieving
  windows
- Register/unregister windows when they are created/destroyed.
API Fixes: Register/unregister created/destroyed tabpages

- Add the 'handle' field to `tabpage_T`
- Add declare/implement functions for registering/unregistering/retrieving
  tabpages
- Register/unregister tabpages when they are created/destroyed.
API Fixes: Generalize buffer, window and tabpage types/functions

- Extract remote types definitions into a macro
- Extract msgpack_rpc helper functions for remote types into a macro
API Fixes: Add macro infrastructure for typed arrays

- Add macros supporting typed arrays in the remote API
- Refactor StringArray-related functions on top of the new macros
API Fixes: Change the integer type of remote objects to uint64_t
API Fixes: Return handles instead of indexes

- Define specialized arrays for each remote object type
- Implement msgpack_rpc functions for dealing with the new types
- Refactor all functions dealing with buffers, windows and tabpages to
  return/accept handles instead of list indexes.