Add `-d` option for `mruby` and `mirb`.

Because `mirb` keeps top-level environment across each iteration.

If `str` and `str2` are the same string object `str->ptr` may be
rewritten by `mrb_str_modify()`.

The return value (void*) may be NULL. Avoid using `mrb_get_datatype()`
directly and use `io_get_open_fptr()` instead.

This was caused by a patch from #2684.

Set the mruby-1.4.1 release date to `2018-4-27`.

Fix heap use after free on mruby-aws-sigv4.

Fix wrong the number of marked objects of MRB_TT_ENV in gray mark phase

If MRB_TT_ENV has stack shared flag or bidx flag, flags is too large
than the real stack size.

Fix array replace leak error in mruby-uri.

sort method should not consume system stack.

Fix undefined error of pointer dereference.

Avoid using `mrb_yield` in C code. The function is not recommended.
Because it doesn't work well with fibers.

`String#lines` (with a block) is now implemented in Ruby.

Otherwise the duplicated object may have shared entities that
should not be modified in-line.

When arguments `copy` and `orig` are the same object.

Fallback to float when compiled binary with 64bit compiler.

closes #3997.