feat: add defer to the Compiler interface

[ivokub]

See discussion in #472. This PR adds PreCompileHook(cb func(API) error) method to the frontend.Compiler. The intent of the method is to register all callback which should be called after the Define method of the circuit is called. These callbacks can be used by gadgets to safely defer any necessary closing tasks (e.g. range checking builds actual constraints, permutation networks build the permutations etc.) without requiring any interaction from the user.

I don't like the implementation though, I think it is really fragile. However, the problem is that I do not want to make frontend.Compile interface any more complex than it barely should, and this also means that there isn't a method to get all the callbacks which the frontend.Compile method can use. That is why I have embedded calling the callback in every builder's Compile method. But this is really really ugly solution.
A better way would definitely be to make list of all callback available to frontend.Compile without making frontend.Compiler interface bigger. One option, for example, would be to use the key-value store with some internal key. For example the implementation for registering callback:

// type CallbackKey is in frontend/internal/XXX

func (b *builder) PreCompileHook(cb func(frontend.API) error) {
    cbs := b.GetKeyValueStore(CallbackKey{})
    b.SetKeyValueStore(callbackKey{}, append(cbs, cb))
}

and then in frontend.Compile:

function Compile() {
// ...
    cbs := builder.GetKeyValue(CallbackKey{})
    // call all callbacks
    builder.Compile()
}

[gbotrel]

follow up on our discussion; my vote is to call it api.Compiler().Defer(...) . I think it's OK to assume users calling a method on a compiler object should read the doc (ie that Defer on the compiler object is not limited to the scope of a function like go defer ) .

[ivokub]

follow up on our discussion; my vote is to call it api.Compiler().Defer(...) . I think it's OK to assume users calling a method on a compiler object should read the doc (ie that Defer on the compiler object is not limited to the scope of a function like go defer ) .

Makes sense. I also think it seems nicer.

I'll also try to append all the deferred methods into the key-value storage to allow frontend.Compile to call the callbacks directly instead of doing in builder.

[gbotrel]

lgtm, w/ PreCompileHook(..) --> Defer(...)

[ivokub]

lgtm, w/ PreCompileHook(..) --> Defer(...)

Renamed, but the implementation still doesn't feel right. I'll try with the kvstore to see if it works better. But need to rebase on unmerged PR.

[ivokub]

It is done but rebased on top of #480 (kvstore PR, not yet merged). Imo looks a bit nicer and I think a bit more future-proof.