Crate co_managed

create managed sub coroutines. managed sub coroutines will be cancelled when the parent exit this is some like the scoped coroutine creation, the difference is that we manage the sub coroutines in a hash map, so that when sub coroutine exit the entry will be removed dynamically and parent doesn’t wait it’s children exit

Structs

• Manager
• SubCo
  represent a managed sub coroutine