Add publicly exposed engine's API for C FFI (including C header)

[lunacys]

We plan to write a fully featured client in C#, so we need to create a dynamically linked library with engine's API.
An example is available here.

The API should be in a separate crate named c-api.

Maybe #10 should be done first so the API is asynchronous.

API:

• Game Settings:
  • Gravity
  • Board
  • Attack
  • Wall Kick Mode
• Time Manager
  • new()
  • update(delta_seconds)
  • reset()
  • enable()
  • disable()
• Cell Holder:
  • get_occupied_cell_count()
  • clear()
  • get_row(y)
  • set_row(y, row)
  • move_down(from_y, should_update_cell_count)
  • move_up(should_update_cell_count)
  • get_cell_at(x, y)
  • set_cell_at(x, y, cell)
• Piece (current piece):
  • get_type()
  • get_cell_type()
  • get_offset_type()
  • get_bounds()
  • get_x() and set_x()
  • get_y() and set_y()
  • rotation_state()
• Piece Manager:
• Piece Queue:
• Board:
  • new(game_settings, wall_kick_data, seed)
  • update(time_mgr)
  • move_left(delta)
  • move_right(delta)
  • rotate(direction)
  • try_hold_piece()
  • get_hold_piece()
  • exec_action(action)
  • hard_drop()
  • soft_drop(delta)
  • push_garbage(amount, messiness)
  • attack(damage)
  • find_nearest_y()
  • reset()
  • enable()
  • disable()
  • get_cell_holder()
  • get_piece_manager()
  • get_current_piece()
  • get_piece_queue()
  • get_layout()
  • get_nearest_y()
  • get_piece_points()
  • get_garbage_queue()
  • get_stats()
  • get_settings()
  • is_dead()

[lunacys]

No longer needed as the client will be rewritten in Rust+Bevy