Cannot destroy a busy fiber

[vtereshkov]

Fibers are unable to free the allocated heap if their functions have not returned in a regular way. This caused memory leaks, such as #16 and #255. To make the behavior more predictable and easier to analyze, any attempt to free a fiber variable whose function has not returned is now treated as an error:

fn main() {
        for i := 0; i < 10000; i++ {
                ctx := new(int)
                resume(make(fiber, |ctx| {
                    a := make([]int, 10000)
                    resume()
                }))
        }                                        // Runtime error: Cannot destroy a busy fiber
}

It is, however, very inconvenient for the user to care about how to terminate every fiber properly.