introduce incremental_gc_step()
rename mrb_garbage_collect() to mrb_full_gc()
gc: replace comment "a round of GC" to "a GC cycle"
Fix mrbc error for space included path
introduce gc_mark_gray_list(), and cleanup redaunt code with it
rename variable_gray_list to atomic_gray_list
fix warning
suppress the warnings from get_valid_iv_sym()

The parameter iv_name SHOULD be string or symbol, but there is no
garantee on it.
I fix typos in variable.c.
Merge pull request #1405 from kyab/fix_mrbc_path_space

Fix mrbc error for space included path
Merge pull request #1406 from Fleurer/gc-mark-gray-list

introduce gc_mark_gray_list(), and cleanup redundant code with it
Merge pull request #1407 from Fleurer/gc-introduce-incremental-gc-step

separate out `incremental_gc_step()` from `mrb_incremental_gc()`, and misc minor rename
Merge pull request #1408 from Fleurer/fix-warning-get-valid-iv-sym

suppress the warnings from get_valid_iv_sym()
Merge pull request #1409 from suzukaze/fix-typos-in-variable.c

I fix typos in variable.c.
restore once removed mrb_garbage_collect()
revise gc_mark_gray_list() not to cause SEGV on Ubuntu 32bit
remove an unused local variable