Should().Throw and Should().NotThrow throw for Func<T>

[krajek]

Closes #422.

This is still work in progress but if you would like to do an early review I would appreciate.
Implemented according to @jnyrup idea of returning AndWhichConstraint from NotThrow which allows continued assertions like this:

//-----------------------------------------------------------------------------------------------------------
// Arrange
//-----------------------------------------------------------------------------------------------------------
Func<int> f = () => 12;
 //-----------------------------------------------------------------------------------------------------------
// Act / Assert
//-----------------------------------------------------------------------------------------------------------
f.Should().NotThrow().Which.Should().Be(12);

All the commented code and tests are part of the WIP.

[jnyrup]

To align with Action assertions we should add
public AndWhichConstraint<FunctionAssertions<T>, T> NotThrow<TException>
public ExceptionAssertions<TException> ThrowExactly<TException>

[krajek]

Ready for the merge with squash.

To align with Action assertions we should add
public AndWhichConstraint<FunctionAssertions, T> NotThrow

NotThrow<TException> does return void, because it does not guarantee that no exception was thrown and that assertion can be continued.

[jnyrup]

Looks good 👍

I didn't thought about the difficulty of the return type for NotThrow<TException>().
For now I agree with returning void.

At some later point I would like if we could come up with a nice way to represent the three possible outcomes:

• didn't throw any exception => success and pass on result for chaining further assertions.
• threw an exception but it was not the unexpected => success but no resulting value.
• threw an exception and it was the unexpected one => fail.

[krajek]

@jnyrup tests you requested are added, it was a good idea.

Additionally, FunctionAssertions now inherits from ReferenceTypeAssertions. It was not extremely important to do it, but at least it is consistent with ActionAssertions among others.

Ready for a merge with squash.