Use non-enumerable Symbols for BLeak-internal properties

[jvilk]

We currently add special BLeak properties to some objects. These are enumerable and visible to the instrumented program.

We should switch to using non-enumerable Symbols so the program cannot accidentally mess with our instrumentation.

This is low-priority as I have not encountered this issue in practice.

[jvilk]

I've made a number of these non-enumerable now.