Refactor assertions

* Use our own AssertionError class. Now we can track the assertion
rather than abusing the operator field. We can also stop depending on
`core-assert` once we implement `t.throws()` ourselves. Reserve the
`statements` property for power-assert output

* Directly create `AssertionError`s, making it easier to specify the
varying options

* Default the assertion message to an empty string, removing workarounds
in the mini and verbose reporters

* Improve `t.plan()` assertion errors. Specify 'plan' as the assertion,
and use `===` as the operator

* Refactor `t.throws()` and `t.notThrows()`, improving readability and
fixing #1227

* Move knowledge of when to show output into `serialize-error.js`. It
sat kind of awkwardly in `assert.js`

* Always try to format errors in mini and verbose reporters, check
if actual / expected values are available when formatting

* Wrap assertions in `assert.js` for use with the `ExecutionContext`

* Only enhance the power-assert assertions, don't use `empower-core`
to wrap the others

* Remove superfluous `empower-core` 'originalMessage' handling. Our
assertions create `AssertionError`s with the correct message

* Refactor overloaded `Test#_assert()` method and assertion tracking
internals. Note that only `t.throws()` and `notThrows()` are (possibly)
asynchronous

index.js.flow

@@ -63,7 +63,7 @@ type AssertContext = {
 	};
 	// Assert that function doesn't throw an error or promise resolves.
 	notThrows: {
-		<U>(value: PromiseLike<U>, message?: string): Promise<U>;
+		(value: PromiseLike<mixed>, message?: string): Promise<void>;
 		(value: () => mixed, message?: string): void;
 	};
 	// Assert that contents matches regex.