* 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
ISLE Fuzz Targets
These are separate from the top-level wasmtime/fuzz fuzz targets because we
don't intend to run them on OSS-Fuzz. They are just for local ISLE hacking.