[RFC] Constify text enc args fix mb string2cells

This PR contains 2 things:

- constify the arguments of some text encoding functions, these didn't change their arguments and it make the contract clearer, as well as allowing the use of `const` strings which I hope we'll be able to use more and more in the codebase
- fix `mb_string2cells`, see: https://github.com/neovim/neovim/commit/677d30d7966dd2766bbf20665791c568dacc427a