gas: add a mechanism for the user to configure which instructions are side-effectful and which ones aren't

[nagisa]

They may want to specify that e.g. some pure instructions are impure in their runtime for some reason (maybe they use a host function call internally to implement it?) or perhaps, and more usefully, they might want to specify that some of the instructions that are blanket impure today are actually pure (or just don’t care for precise gas at the point in time).