Removing lifetime parameter for Executor and LocalExecutor

[GreyGoo]

What is the point of it? It is not reasonable to statically track the lifetime of Executor which only contain a Arc.

[Taiki Endo]

IIUC, this is needed to spawn a non-static future: f9e28cd

async-executor/src/lib.rs

Lines 68 to 69 in dacd4db

 /// Makes the `'a` lifetime invariant. 
 _marker: PhantomData<std::cell::UnsafeCell<&'a ()>>, 

async-executor/tests/drop.rs

Line 76 in dacd4db

 let task: Task<&str> = ex.spawn(async { &*s }); 

See also safety requirements of async_task::spawn_unchecked