Minimize/fix function declarations
Remove unused mark_current_freelist function
Give SHARED_LIST_SIZE a CGC_ prefix
Stop using printf in signal handler

1) Too verbose for GC heavy tests
2) Occasional deadlock on OS X
Change metor benchmarks to silence warning

ruby 2.0 warns about using class variables from top level.  Change
them to be globals.
Track GC runs for GC.stat