Manage assertions through a class

[qlonik]

This changes assertions to be properties on the class. Before,
assertions were functions which needed to be bound to the test instance.
This led to the fact that snapshot() assertion accessed property which
only existed on Test - to compare snapshots. Now, the functions which
assertions access are clearly defined as those that need to be
implemented by child class.

This change also updates how ExecutionContext is created, as well as
minor changes to the test/assert.js.

Additionally, if this was to be changed to typescript, properly typing _skippable, _enhanced and parameters to all assertion functions, we would get type information of all assertions methods on t for free. Also adding doc comments to all assertions would give documentation which lives near the source code.

There are also some minor type related changes:

1. In throw assertions, there is no longer arguments.length passed to validateExpectations(). Instead validateExpectations checks if second argument null or undefined.
2. In some places there is isPromise() being used, however it only checks if .then on that object exists and it is a function. So, in those cases that PromiseLike object is cast to Promise with .resolve().

[novemberborn]

I'm reluctant to expose the protected methods on the execution context. Folks will discover them and use them and I'd rather they didn't. Perhaps we can utilize a WeakMap instead?

[qlonik]

I'm reluctant to expose the protected methods on the execution context. Folks will discover them and use them and I'd rather they didn't. Perhaps we can utilize a WeakMap instead?

This is a good idea. How about those protected methods on Assertions class? Since ExecutionContext now extends that class, the private methods starting with _ might also be discovered and used. Should those private methods be hidden via WeakMap too? If we hide them, then it is slightly more difficult to extend the base Assertions class, as we would have to pass those private functions to the constructor of Assertions and children classes cannot simply overwrite those methods.

[novemberborn]

This is pretty close to landing, nice work @qlonik!

[novemberborn]

🎉

I'll land #2096 tomorrow and fix the merge conflicts here, and then I'll land this one.