separate out `incremental_gc_step()` from `mrb_incremental_gc()`, and misc minor rename

hi Matz and Nari,

I hope to digg some performance optimization for mruby's GC in the future, but before that work, I'd like to make some minor refactors & comment-fix incrementally. If there is anything wrong in my future PRs, please let me know :)

Thanks 

- introduce `incremental_gc_step` from `mrb_incremental_gc()`, separate the logic on calculate step size outside.  
- rename `mrb_garbage_collect()` to `mrb_full_gc()`, in comparison to `mrb_iemental_gc()`
- minor comment fix: "a round of GC" to "a GC cycle"