fix typos
Doesn't clear black of all marked objects in major collections.
Since the minor collector doesn't need to mark these objects, the worst-case pause time is reduced.
Initialize edges of RRange.
Modify `Range#initialize` to accept optional `exclusive` argument.
Merge pull request #728 from takkaw/khash_typo

fix typos
Merge pull request #729 from authorNari/unclear-black-in-major-collector

Doesn't clear black of all marked objects in the major collector.
Merge pull request #730 from masamitsu-murase/modify_range_initialize

Modify `Range#initialize`