Cannot destroy a fiber that did not return

[vtereshkov]

Fibers are unable to free the allocated heap if their functions did not return 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 did not return is now treated as an error:

fn foo(parent: fiber, ctx: ^int) {
        a := make([]int, 10000)
        fibercall(parent)
}

fn main() {
        for i := 0; i < 10000; i++ {
                ctx := new(int)
                fibercall(fiberspawn(foo, ctx))
        }                                        // Runtime error: Cannot destroy a fiber that did not return
}

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