EnterGuard<> must always be used

[farazshaikh]

**Version
tokio v1.3.0 (*)

Platform
all

Description
{
handle.enter(); // IMO Never makes sense to immediately drop the handle.
foo..
}

[short summary of the bug]
For "some" reason we ended up creating 2 runtimes in a project.
We used .enter() to specifically tie some code down to one of specific runtimes.
Missing the ```let _guard = `` part didn't get flagged by the compiler.
Overall caused some confusion as the code rightly didn't work as executed.

runtime {
   let nested_runtime = runtime::new
   nested_runtime::enter();   // missed the let binding. Intention was to use the nested runtime.
}

I expected to see this happen:
The compiler should have warned, linter should have warned. Code should not have compiled.

Instead, this happened:
Code compiled and didn't worked as intended.

[Darksonn]

Closed by #3609