* gc.c (rb_objspace_t::heap_pages): rename field names:
  * used -> allocated_pages
  * increment -> allocatable_pages
  * length -> sorted_length
  And remove unused `limt' field.
* gc.c: rename macros:
  * heap_pages_used -> heap_allocated_pages
  * heap_pages_length -> heap_pages_sorted_length
  * heap_pages_increment -> heap_allocatable_pages
* gc.c (gc_stat_internal): fix symbol names
  ref: [Feature #9924]
  https://docs.google.com/spreadsheets/d/11Ua4uBr6o0k-nORrZLEIIUkHJ9JRzRR0NyZfrhEEnc8/edit?usp=sharing
  Yellow color fields in this table are changed.
* test/ruby/test_gc.rb: catch up this change.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47471 b2dd03c8-39d4-4d8f-98ff-823fe69b080e