add id_function

[cscherrer]

This allows solve! to take a function argument, so now it's solve!(id_function, ast; toplevel=true). Here id_function is a function of two arguments,

id_function(::Ref{Bool}, ::Symbol)

If the function is left out, it's taken to be (a, _) -> a

[cscherrer]

Any thoughts on this @thautwarm? Using a Ref as an ID is efficient, but there are a lot of cases where we'll need something different. We could walk the AST after calling solve!, but it would be nice to avoid this extra overhead if possible.

If you like this idea I could merge it here, so it would be added to the PR JuliaStaging#29

[thautwarm]

I think such design is good: it gets decoupled from the design mistake of NameResolution.jl.

The only issue is the compatibility. Merging it could be bring breaking changes, and I'm not ready for it.

Maybe you can use different Var when id_function is specified, and the default id_function create scoped Expr with the original Var?

[cscherrer]

The only issue is the compatibility. Merging it could be bring breaking changes, and I'm not ready for it.

That's ok. I can work around it, this was just an idea for some added convenience.