Fix tracing of allocations from C

[chrisseaton]

We get failures while tracing allocations from C extensions. I think this fixes it, but I'm not keen to try to architect a test case, to be honest.

[eregon]

In general I feel ObjectSpace.trace_object_allocations is crazily expensive, and something we should probably give a performance warning for.
CRuby docs also say:

Note that this feature introduces a huge performance decrease and huge
memory consumption.

So I think of it as something that should only be used for debugging.
Also notes it invalidates a lot of code when enabling/disabling allocation tracing.

One thing we could do to make it a little bit less horrible would be to store allocation information on the DynamicObject iself, instead of on a separate Hash which leaks all allocations done while enabled.
=> But with trace_object_allocations_clear that is not so simple.

Self-note: the memory_profiler uses ObjectSpace.trace_object_allocations_start

[eregon]

The docs of this say:

This method is only expected to work with C Ruby.

But it seems we already implement it.
Anyway I think adding specs for it is fine, it doesn't seem worse than ObjectSpace.each_object.

[chrisseaton]

We implement quite a bit more of objspace in random ways.

[eregon]

ObjectSpace.memsize_of_all.should > 0 seems more expressive and clearer, I'll change to that.

[chrisseaton]

trace_object_allocations_clear could set a generation flag, and allocation information would be tagged with the current flag.

But as you say, not really worth improving it unless a need is seen. I'm fixing a crash here, that's all.

[eregon]

trace_object_allocations_clear could set a generation flag, and allocation information would be tagged with the current flag.

I had the same idea and implemented that.