Allow for configuring costs per WebAssembly opcode

[ureeves]

This is not meant to merged, as we maintain this crate slightly differently, but it is important enough functionality to require review.

If the commits in this PR are included, the user of dusk-wasmtime will be able to configure the fuel costs of each WebAssembly opcode, in much the same way as we could before. This paves the way for making storage cost more than other opcodes, as required by dusk-network/piecrust#359

[moCello]

If I understand correctly, with this change we could set the amount of "fuel units" for the call of a single op-code individually to any value we like?

[HDauven]

If I understand correctly, with this change we could set the amount of "fuel units" for the call of a single op-code individually to any value we like?

That's right. If you look at the last line in tunables, it calls the for_each_operator! macro from wasmparser, and applies the define_operator_cost macro to it, which uses the default_cost macro.

It iterates over all the operators that wasmparser recognizes and assigns the default fuel cost of 0 for the defined ops and 1 for others. In the default_cost macro we can define custom gas cost for specific ops.

[moCello]

LGTM

[ureeves]

As said, this won't be merged, but I'm happy you like the implementation