zsh completion requires sourcing and can't get autoloaded easily

The current version of the zsh completion does not support zsh autoloading.

This can be repaired by removing the wrapping function (removing first and last line basically) and naming it `_exercism`. One can simply throw it into the `$fpath` and add `autoload _exercism` to the `.zshrc`.

Then zsh would only try to load the function if one tries to complete an argument to exercism.

Can we change the zsh completion to be more autoload friendly?