with_locals can be very expensive to run, and we've found our software can get into trouble when exceptions with complex locals are triggered in a tight loop, however, the information contained in the locals is often extremely useful for debugging. This commit allows the application to specify a means of selecting if locals should be captured, with a sample method which captures only the first for a given traceback. The cost of hashing the stack is quite small compared to the cost of stringifying the locals. Clearly the code can leak quite badly if used in a project with code generation, but in general the set of tracebacks should be small.

Note this can be quite awkward as Sentry doesn't really have a means of searching for a capture with locals, though one can normally just hunt backwards for the first instance.