Fix and optimize Task

This release introduce the following changes:

• Create missing Run method overloads from Task class.
• Create stub for Current property from TaskFactory class.
• Fix TrySetResult method from Task class.
• Optimize Task promises: Delay, WhenAll and WhenAny.
• Optimize Task wait methods: WaitAll and WaitAny.

Support Contracts and WebRequest timeout, and fix ValueTask awaiters

This release introduce the following changes:

• Fix implementation of ValueTask awaiters;
• Fix to unwrap exception of nested Task when using Run method of Task class;
• Support contracts (System.Diagnostics.Contracts);
• Support timeout on WebRequestAsyncExtensions.