Experimental support for heap traversal

[wks]

This showcases the experimental heap traversal support introduced in mmtk/mmtk-core#1174 and mmtk/mmtk-ruby#90

Calling GC::MMTk.print_all_objects will simply print all object references in the heap.

This PR also enables TracePoint. The following snippet will be able to print "new line!" before each line.

tp = TracePoint.new(:line) do
  puts "new line!"
end

tp.enable

puts "hello"
puts "world"

Output:

new line!
hello
new line!
world

[wks]

#91 has been merged into the dev/mmtk-overrides-default branch, and has enabled ObjectSpace::each_object and TracePoint. The dev/mmtk-gc branch also has its own wrapper for the object-enumeration feature from mmtk-core. This PR can be closed.