Add some table functions

- Add table.pack
- Add table.insert
- Add table.remove
- Add table.sort
- Change ipairs to make it return 3 values, iter, i and v.
- Add Lua and pico8 tests for table.pack, table.insert, table.remove, table.sort and return values for ipairs.
- Add comments to let the user know about getmetatable and rawget dependencies.
- generate_pico8.py now exports missing.lua in 1-space indentation.
- generate_package.py now exports the "table" table.

Rename toolbelt to missing

Add background image for template_toolbelt.p8

* Convert to lowercase template_toolbelt.p8
* Lowercase string to replace in generate_pico8.py

Update README.md to add precision on the license.

Update README.md to change title.