Don't dump object internals

[jonasdt]

This improves #402 as follows:

• Only dump object internals if a custom caster exists. This allows integrations with frameworks to define custom casters. E.g. for Laravel we can dump the attributes of an Eloquent model.
• Don't dump verbose information (e.g. exception stack traces).

[rossbearman]

This change makes a dramatic speed improvement, from around one minute to under a second, assuming this doesn't cause any issues it would be great to see this pulled into the core.

[denis-sokolov]

Thank you, @jonasdt!