WIP: Allocation tracing

[jhawthorn]

I want to add the ability to trace and query allocation information about specific objects in the same way that ObjectSpace.trace_object_allocations works, except with full stack information.

obj = nil
allocations = Vernier::AllocationTracer.trace do
  obj = Object.new
end
stack = allocations.stack(obj)
puts stack
# Class#new at <cfunc>:0
# block in <main> at /Users/jhawthorn/src/vernier/test_allocation_tracing.rb:5
# Vernier::AllocationTracer#trace at /Users/jhawthorn/src/vernier/lib/vernier/allocation_tracer.rb:19
# Vernier::AllocationTracer.trace at /Users/jhawthorn/src/vernier/lib/vernier/allocation_tracer.rb:13
# <main> at /Users/jhawthorn/src/vernier/test_allocation_tracing.rb:4

TODO: retained profiling mode should be built off of this functionality