* Upgrade all crates to the Rust 2021 edition
I've personally started using the new format strings for things like
`panic!("some message {foo}")` or similar and have been upgrading crates
on a case-by-case basis, but I think it probably makes more sense to go
ahead and blanket upgrade everything so 2021 features are always
available.
* Fix compile of the C API
* Fix a warning
* Fix another warning
* Finished the Markdown Parser Example for Wasmtime
* Made requested changes
* Tiny change to explanation of `--dir` CLI arg
* Add `bash` annotations to shell script code blocks
* Trying to fix the markdown example bug
* Figured out rustdoc, and what needed to be done
* Made requested changes
Co-authored-by: Till Schneidereit <till@tillschneidereit.net>
