Add cost fn hash to caching path

[chipshort]

Now I was wondering if/how we can turn this function into something we can hash and add to the caching path. Either by serializing the function (some scripting language) or serialize the op -> cost as a table/map. Could you create a ticket for that as mid or long-term improvement?

Originally posted by @webmaster128 in #2204 (comment)

[webmaster128]

How cracy would it be to implement fn cost in Wasm?

[chipshort]

A little bit crazy. I wouldn't do that just to get a hash of it.
One thing I've been thinking about is using a derive macro that just hashes the incoming source code.

[webmaster128]

How can we ensure that changes in is_accounting_operator (implemented in the metering middleware) would lead to a different hash? Can we get hash values from normal functions or would it need to be implemented as a macro?

[chipshort]

Hm, that's a tough one. The macro can only access the code that's annotated with it.
But generally, this shouldn't be a problem for us, since we bump the serialization version with each wasmer upgrade anyways (if we want to get really crazy, we can even have a macro that reads our Cargo.toml and hashes the wasmer version).

[chipshort]

So, basically there is https://docs.rs/code-hasher/0.1.0/code_hasher/ and https://docs.rs/include-cargo-toml/0.1.0/include_cargo_toml/, but we could also implement our own, since both of these are very small.

[webmaster128]

Sounds great. Happy for any implementation that works and that we can replace. So the idea here is to hash together 1. the fn cost source code and wasmer-middlewares version?

[chipshort]

Yes, exactly.

[chipshort]

Another note: We should make sure to backport this to all supported versions when it is implemented.