EVM on WASM: Improve account storage performance

Context

Current owner: @rodibozman (just rebased the work done by Thomas P. and added a few tweaks/refactorisation)

@thomas.pecseli:

A long time ago, when the account storage module was first written, we had to do a call to store_has before doing reads from a location to avoid some errors. Now, the store_read does this check for us and returns a Err(RuntimeError::PathNotFound) if there is nothing to read. Therefore we end up doing to host calls to store_has. This MR should improve performance and bring us up to date with the kernel runtime.

Also, there was a bug in how the kernel checked for contract code existence.

Manually testing the MR

Use the CI/unit tests as usual.

Checklist

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