Implement generic assert_eq

[bufferhe4d]

Adds generic assert_eq using #231.

It recursively goes through any nested structure like arrays or structs.

[katat]

This is really nice to have! Great work @bufferhe4d !

[katat]

@mimoo might want a round of review

[mimoo]

let's merge this one?

[mimoo]

why return a comparisons array btw? Why not just do the constraints right here?

match (lhs_cvar, rhs_cvar) {
        // two constants
        (ConstOrCell::Const(a), ConstOrCell::Const(b)) => {
            if a != b {
                Err(Error::new(
                    "constraint-generation",
                    ErrorKind::AssertionFailed,
                    span,
                ))?
            }
        }

        // a const and a var
        (ConstOrCell::Const(cst), ConstOrCell::Cell(cvar))
        | (ConstOrCell::Cell(cvar), ConstOrCell::Const(cst)) => {
            compiler.backend.assert_eq_const(cvar, *cst, span)
        }
        (ConstOrCell::Cell(lhs), ConstOrCell::Cell(rhs)) => {
            compiler.backend.assert_eq_var(lhs, rhs, span)
        }

[bufferhe4d]

Sorry for the delayed reply. I think the reason was that I couldn't figure how to pass the compiler backend recursively. Hence used the table instead. I can look into it again to make it more elegant.