This reverts commit b8869498.

fix env->stack misadjusting: fix #4029

`Hash#clone` did not work properly; fix #4030

let Hash#merge keep ifnone value

Since `TT_ICLASS` is a internal object that should never be revealed
to Ruby world.

`exc_debug_info()` and `mrb_keep_backtrace()` raise `FrozenError`
if the exception is frozen and lead to infinite loop.

`then' was added in CRuby 2.6.

Fix test of mruby-pack with big-endian CPUs

When running the mruby-pack test with big endian, test data is incorrect,
so it will fail with "i" and "I".

------
Fail: pack/unpack "i" (mrbgems: mruby-pack)
 - Assertion[1] Failed: Expected to be equal
    Expected: "\xff\xff\xc7\xcf"
      Actual: "\xff\xff\xcf\xc7"
 - Assertion[2] Failed: Expected to be equal
    Expected: [-12345]
      Actual: [-14385]
Fail: pack/unpack "I" (mrbgems: mruby-pack)
 - Assertion[1] Failed: Expected to be equal
    Expected: "\x00\x0090"
      Actual: "\x00\x0009"
 - Assertion[2] Failed: Expected to be equal
    Expected: [12345]
      Actual: [14640]
------

This will fix the test data at big-endian.
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

Add `-r` option for `mruby` and `mirb`. (mrbgem is not supported)

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.