Create trait for builtins

[mimoo]

we should standardize how we write builtins by using some trait that looks like this or something:

trait Builtin {
    const sig: &'static str;

    const no_typecheck: bool;

    fn typecheck(
        generics: &GenericParameters,
        vars: &[VarInfo<F, LinearCombination<F>>],
        span: Span,
    ) -> Result<()>;

    fn builtin(
        compiler: &mut CircuitWriter<R1CS<F>>,
        _generics: &GenericParameters,
        vars: &[VarInfo<F, LinearCombination<F>>],
        span: Span,
    ) -> Result<Option<Var<F, LinearCombination<F>>>>;
}

and the typecheck() function could contain all of the actual checks that need to be done.