refactor(context): reduce responsibilities taken by Context, ClassContext, ExampleBase

[BrainCrumbz]

Main purpose of this PR is to streamline the logic behind:

• building chains of method level hooks;
• running chains of hooks through ancestor levels,
• running the example itself,
• setting example exception.

At the same time, this happens while extracting those responsibilities out of Context, ClassContext and ExampleBase, and moving those to dedicated components.

Some results:

• Context is much shorter now and more approachable
• repeated code related to discovering method-level hooks is now shared in a single base class
• repeated properties and methods related to running hooks through ancestor levels are now shared in a base class or two
• determining whether a hook can run or not is always in a dedicated CanRun method
• exception collected running through a hook chain is always available in a AnyException method
• all responsibilities related to running an example in its context, assigning right exception, writing outcome to formatter, are all to be found in a dedicated RunnableExample class
• write access to properties in Context, ClassContext and ExampleBase is protected, to ease understanding what can be modified outside of class