gc.c: add minor marking and lazy sweeping options to GC.start

* gc.c (gc_start_internal): GC.start() now accepts two optional
  keyword arguments. These can be used to disable full_mark (minor
  mark only) or disable immediate_sweep (use lazy sweep). These new
  options are useful for benchmarking GC behavior, or performing minor
  GC out-of-band.
* test/ruby/test_gc.rb (class TestGc): tests for new options.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44027 b2dd03c8-39d4-4d8f-98ff-823fe69b080e