refactor: Worker is not a Future (#7895)

This commit rewrites deno::Worker to not implement Future
trait.

Instead there are two separate methods:
- Worker::poll_event_loop() - does single tick of event loop
- Worker::run_event_loop() - runs event loop to completion

Additionally some cleanup to Worker's field visibility was done.

cli/coverage.rs

@@ -1,7 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
 
 use crate::colors;
-use crate::inspector::DenoInspector;
 use crate::inspector::InspectorSession;
 use deno_core::error::AnyError;
 use deno_core::serde_json;
@@ -14,8 +13,7 @@ pub struct CoverageCollector {
 }
 
 impl CoverageCollector {
-  pub fn new(inspector_ptr: *mut DenoInspector) -> Self {
-    let session = InspectorSession::new(inspector_ptr);
+  pub fn new(session: Box<InspectorSession>) -> Self {
     Self { session }
   }