Remove Valgrind task and suppression file

* The suppression file is for an unsupported version of Ruby
* The task behaves differently from just running Valgrind on the command
  line.
Move test for GLib::MainLoop#run to unit tests
Remove incorrect method setup
Fix typo
Explain RuboCop AccessorMethodName configuration
Add to TODO
Include gemspec in the gem
Remove old test_files gemspec attribute
Provide config.h for test libs
Remove Rubinius-specific gems from Gemfile

These are no longer needed since Rubinius 2.2.5.
Add allocation_tracer gem
Use method name as is when generating its stub
Avoid extra argument array unpacking
Add configuration for PullReview
Two performance-related tweaks [merges 'performance-experiments']