Interpreter stackmap needs cleanup

[kg]

stackmap.cpp has a simple cache for 'stack maps' which describe the layout of a given class on the interpreter stack. We use this to do proper GC reporting of locals.

It has a few problems that need to be fixed:

• The thread-local cache isn't cleaned up when a thread is closed.
• Once we're done compiling the cache data persists forever, which means it will gradually get bigger and bigger as the app runs. This could be a big problem in interp-only mode.

The right solution for now is probably to migrate it to the InterpCompiler.

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