Add cargo-fuzz target for OSS-Fuzz integration

[canolgun-commits]

Adds cargo-fuzz target for continuous fuzzing via OSS-Fuzz.

[cfallin]

This is a severe violation of our trivial PR policy, and also makes no sense:

• We already have many fuzz targets, and we are already on OSS-Fuzz and have been for many years;
• This PR deletes all of our existing fuzz targets (!!!);
• This PR adds a trivial fuzz target that... parses a string as a string?

This is either (i) an unusually transparent attempt at karma-farming by getting a trivial/meaningless contribution to an open-source project, or (ii) a malicious attempt at reducing Wasmtime's security by deleting all of our fuzz targets. As such, I will close it.