Add test for backquote.
* Implement code generation of NODE_DXSTR.
* Fix NOVAL NODE_XSTR.
Merge pull request #1966 from take-cheeze/backquote

Add test for backquote.
Implement Kernel#define_singleton_method
Add to_enum unless block
Add Array#fetch
Merge pull request #1967 from ksss/define_singleton_method

Implement Kernel#define_singleton_method
Merge pull request #1968 from suzukaze/add-to_enum-unless-block

Add to_enum unless block
Array#reverse_each should return Enumerator; ref #1968
Merge pull request #1969 from suzukaze/add-array.fetch

Add Array#fetch
Merge branch 'master' of github.com:mruby/mruby
Fix a typo
Kernel#send should preserve nregs in callinfo; close #1970
Merge pull request #1971 from suzukaze/fix-typo

Fix a typo
Enumerator#initialize support duck-type
Use mrb_intern_lit instead of mrb_intern in test driver.
Merge pull request #1974 from ksss/enumerator-duck

Enumerator#initialize support duck-type
Merge pull request #1975 from take-cheeze/driver_mrb_intern_lit

Use mrb_intern_lit instead of mrb_intern in test driver.
Add Array#fill
Merge pull request #1978 from suzukaze/add-array.fill

Add Array#fill
revert #1974; ref #1979