Detect cycles in dependencies of decls in modules

[kevinbarabash]

Since decls in modules should all be pure, they can be executed in any order and depend on each other in any way as long as they don't introduce cycles, e.g.

let a = b();
let b = fn () => c + 5;
let c = 10;

is valid, but

let a = b();
let b = fn () => c + 5;
let c = a;

is not, because c -> a -> b -> c which makes the decls cyclic.