Make export of non-scalar values for events optional

[sebastianbergmann]

To address performance issues such as #5183 and sebastianbergmann/exporter#18 (comment), we changed the event system to not emit the Test\AssertionSucceeded and Test\AssertionFailed events when they do not have subscribers. The recently opened #5510 shows that there is at least one more performance issue related to the Test\AssertionSucceeded and Test\AssertionFailed events.

I discussed this issue with @theseer today and we came up with a different (hopefully better) solution to the performance issue that is rooted in exporting large object graphs to strings: instead of only emitting the events in question when they have subscribers, we would like to always emit them, but make the expensive export operation optional.

[sebastianbergmann]

@muzcb @Slamdunk Can you have a look whether this reintroduces the performance penalty again?

[sebastianbergmann]

It just occurred to me that we still use sebastian/exporter directly in Constraint::toString().

[sebastianbergmann]

As of 56e2355, Constraint::toString() only exports objects when needed (for assertion failure messages) or when explicitly requested.