Drop Code objects in AOT (PRODUCT && dwarf_stack_traces)

[alexmarkov]

In AOT mode, when vm_service/profiler is not present (PRODUCT/flutter-release build) and --dwarf-stack-traces is used, there is a little need for Code objects. Currently, Code objects occupy significant portion of heap size of objects in the snapshot, so removing them would decrease memory usage and would also reduce AOT snapshot size.

In order to remove most Code objects we need:

• Provide an alternative lookup mechanism to access CompressedStackMaps corresponding to a given PC.
• Remove uses of Code from stack traces in --dwarf-stack-traces mode.
• Remove most uses of ReversePc::Lookup.

For now, we can keep Code objects for exception handling as there is only a small number of codes with exception handlers.