Cascading returns are not collapsed

[dwayne]

Add the following test cases

("return if (true) { return 10; };", Some(Object::Int(10))),
("return if (true) { return if (true) { return 10; }; };", Some(Object::Int(10))),
("return if (true) { return if (false) { return 10; } else { return 5; }; };", Some(Object::Int(5))),

to the test_return_stmt function in src/evaluator/mod.rs to see what I mean.

For e.g. the tests fail as follows:

left: `Some(Int(10))`,
right: `Some(ReturnValue(Int(10)))`

left: `Some(Int(10))`,
right: `Some(ReturnValue(ReturnValue(Int(10))))`

left: `Some(Int(5))`,
right: `Some(ReturnValue(ReturnValue(Int(5))))`

Three possible solutions:

1. Implement Eq for Object such that ReturnValue(ReturnValue(Int(10)) would be equal to Int(10) etc. For e.g. that approach taken here.
2. Add a smart constructor specifically for ReturnValue, say makeReturnValue v, such that if v is already a ReturnValue we simply return v otherwise we return ReturnValue v.
3. Collapse the ReturnValue when the final output from the evaluator is a ReturnValue.

Note: If these examples are tried in the online interpreter they will display the correct result because of how Display is implemented for Object. Hence, it hides the issue.

[dwayne]

A related problem. Given,

[fn (x) { return [x]; }][0](4)[0]

it fails with:

uknown operator: [4] 0

But,

[fn (x) { [x] }][0](4)[0]

without the return statement gives the correct answer.