Tessellator was broken for years! Revert-fixing.

Due to a typo in the cfg flag, tessellator was never built properly (we can even remove it as it is clearly not being used by anyone).
This is a known issues in the rust compiler: https://github.com/rust-lang/cargo/issues/10554
I reverted Lyon to 0.16.2 and it worked fine.
I added a CI nightly check step to make sure this won't happen again

Do we want to keep this and spend the effort to upgrade to the latest Lyon crate, or is this a useless feature and should be removed?

georust / geozero