Unsound problems in may_queue crate

[safe4u]

Hi, thanks for your contribution to this great coroutine library.

However, we have found lots of unsound problems in the util crate may_queue.
For example, the set and get set and get the data without boundary checking for performance consideration.
These functions are actually unsafe since they ask the caller to guarantee the index is in-bound and the compiler fails to check this.

It is certainly ok to mark the unsafe functions as safe and use them with care in 'may'.
But considering the crate may_queue is available in crates.io and these functions are public, we think it's still necessary to make these functions more sound.

Suggestions

Here are some action choices we suggested:

1. Mark these functions as 'unsafe' which would lead to large code changes.
2. (recommended) Add assert! or at least debug_assert! macros to validate that the index is in-bound.