Should have handled the case `to_a` returns `nil`.

Simplify MSVC detection to `mrb_static_assert`

Merge pull request #5062 from shuujii/use-normal-static_assert-in-mrb_static_assert-as-much-as-possible

Use normal `static_assert` in `mrb_static_assert` as much as possible

Use `struct _stat32` instead of `struct __stat32`

It is described as `struct __stat32` in the MSVC reference manual.
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/stat-functions?view=vs-2019
But it doesn't really exist, so it must use `struct _stat32`.

It also replaces `struct __stat64` with `struct _stat64` to make it look
nicer.

* `_Static_assert` can also be used with `-std=gnu99` on GCC >= 4.6.
* `static_assert` can be used on MSVC.
* `static_assert` can be used even on old G++/Clang++ if
  `__GXX_EXPERIMENTAL_CXX0X__` is defined.

Allow `mrb_static_assert()` to be used outside of functions

The use of `struct` is an idea by @dearblue.

Note that the home brew version of `mrb_static_assert` only works within
the function body.  This reverts commit 8f99689b.

Merge mruby 2.1.2

Extend the `cipush()` and `cipop()` functions

Improve `rake benchmark`

- Use POSIX format instead of GNU extension for `time` command.
  For example FreeBSD's `time(1)` does not have GNU extensions available.
- Sort `benchmark/bm_*.rb`.
  This is because the order of the bar graph cannot be uniquely
  determined depending on the result of `Dir.glob`.

- Remove ` kh_put_prepare` function used only internally
- Remove `n_occupied` member from `kh_` struct

`puts` should print newline when called without arguments.

The function corresponding to `ht_hash_func()` was as follows in the days of
khash implementation (before d78acc7a).

  ```c
  mrb_hash_ht_hash_func(mrb_state *mrb, mrb_value key)
  {
    enum mrb_vtype t = mrb_type(key);
    ...
    switch (t) {
    ...
    default:
      hv = mrb_funcall(mrb, key, "hash", 0);
      h = (khint_t)t ^ (khint_t)mrb_fixnum(hv);
      break;
    }
    ...
  }
  ```

When switched to the segmented list implementation (d78acc7a), this function
was changed as follows.

  ```c
  sg_hash_func(mrb_state *mrb, seglist *t, mrb_value key)
  {
    enum mrb_vtype tt = mrb_type(key);
    ...
    switch (tt) {
    ...
    default:
      hv = mrb_funcall(mrb, key, "hash", 0);
      h = (size_t)t ^ (size_t)mrb_fixnum(hv);
      break;
    }
    ...
  }
  ```

Since the argument `t` was added, the variable for type tag was changed from
`t` to `tt`, but the variable used in the expression of `h` remained `t`.

Probably this is an omission of change, so fixed it.

ref c07f24cd and close #5035

Update document for `ObjectSpace.memsize_of` [ci skip]