Replace `unwrap_or` with `unwrap_or_else` and vice versa depending on cost of computing argument

[xa888s]

To fix #35

My editor auto formatted (with rustfmt) the files on save but the (non-whitespace/formatting) changes are quite small. I have finished unwrap_or* methods and now I'll work on the ok_or* methods

[xa888s]

I'm happy to handle rustfmt configuration in a separate PR, as it's an important part of linting along with clippy.

This is a good idea, I think I'll set this aside until we can merge a formatting PR.

[kevinaboos]

Hi @xa888s, it appears that there are still quite a few unrelated formatting changes in this branch. Once you remove those, I am happy to review and accept this PR.

Also, kindly merge the latest changes from theseus_main into this branch, which remove the top-level build.rs script.

[kevinaboos]

Closing due to inactivity and unrelated changes in favor of #641.