Implement Wasmtime's new API as designed by RFC 11. This is quite a large commit which has had lots of discussion externally, so for more information it's best to read the RFC thread and the PR thread.
* Add example of execution limits using fuel consumption
* run rustfmt
* Use a more naive WAT implementation
* Catch error and return cleanly
* Add a C example to demonstrate fuel consumption
* Add error handling for add_fuel