Allow class authors to customize the way debuggers display instances

If a class defines method __toDebugDisplay, the variable serializer will call it when serializing with DebuggerSerialize. The resulting value is added as property __PHP_DebugDisplay to the serialized object. If an object with such a property is serialized with DebuggerDump, the serialization of the property is used as the serialization of the object with the property.

Differential Revision: D954810
Disable quick/debugger/list.php

This test is flaky on some dev boxes. Disabling it until task 2558262 is implemented. That should get rid of the flakiness.

Differential Revision: D957673
provide run mode which runs all tests in mode matrix

Travis only supports running a single commandline to run tests so provide a
mode where we can run JIT+interp X repo+non-repo now that contributors are
starting to submit contributions that only break in repo mode.

Differential Revision: D957147
Fix updateTrackedState for StRef

It was using the wrong src number and always getting nullptr.

Differential Revision: D955583
Updating ext_string for benchmark optimizations

String optimizations, this gives us a 40% reduction in cycles for the revcomp benchmark.

Differential Revision: D934224
Add PHP_DEBUG constant

Just as it says on the tin.

Differential Revision: D958042