std.uni in debug library does invalid things with GC memory

[schveiguy]

in std.uni, the destructor for the CowArray!GcPolicy sets the data in the referenced array to a sentinel value.

However, if this is called from a GC finalizer, the memory referenced may be invalid.

The debug code should be instrumented to check GC.inFinalizer before writing the sentinel. Or we just get rid of the sentinel setting, as it's not that critical of a debug tool to have at the cost of segfaults.