The basic idea of this change is from @dearblue.

Note: the arguments of `mrb_str_pool()` have changed, but the function
is provided for internal use (No `MRB_API`). So basically you don't have
to worry about the change.

Fix ainfo with keyword arguments; fix #4921

Call `va_end()` before return

The behavior when returning from a function without `va_end()` is
undefined.

Add `mrb_num_args_error()` for "wrong number of arguments" error

Revert "SHARED string is not required when sharing POOL string" (75949836)

Because literal pool may be released by GC.

#### Example:

  ```ruby
  s1 = eval('"abcdefghijklmnopqrstuvwxyz01"')
  GC.start
  p s1  #=> "\x00\x00\x00\x00\x00\x00\x00\x90\x00\x00\x00\x00\x00\x00\x00\x90\x03\x00stuvwxyz01"
  ```

Integrate `i` and `arg_i` in `mrb_get_args()`

Do not include `stdint.h` before `mruby.h`; ref #4750

The behavior of these two variables is the same.

Silence GCC warning in `time.c` on Travis CI

Silence the following warnings:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:260:55: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if ((MRB_INT_MAX > MRB_TIME_MAX && i > 0 && i > MRB_TIME_MAX) ||
                                                  ^
  ```

Fix builds for modern mingw; fix #4869

What I intended to fix in #4869 was a patch for an old MinGW.
Recent MinGWs have their own `mkstemp()` function.
I knew this after checking the patch #4903.

Use `rake` instead of `minirake` in `.gitlab-ci.yml`

There was an error in the `.gitlab-ci.yml` generation script, so it was also
fixed.

Avoid unneeded directory creation during `rake -T`, etc.

Use `Rake.verbose` instead of `$verbose` and `$pp_show` in build scripts

The incompatibility that the commands of `FileUtils` origin output verbose
by default due to the changes in d8a5163b and 26e6e75b is also fixed.

Use Rake DSL instead of commands of `FileUtils`

- Respect `--verbose(-v)` and `--dry-run(-n)` options.
- Silence warnings to keyword arguments on Ruby 2.7.

`gcc` sanitizer does not work on some environment (namely my localhost).
Although it works on Travis, it is not convenient that tests fail on the
local host. We keep sanitizer tests on the local (using `clang`).

This reverts commit 9cde9673.

It is stricter than CRuby but confusing anyway.

But it causes warnings as CRuby does; fix #4892 fix #489

The patch was originally written by @take-cheese. I have fixed the
conflict caused by `minirake` deletion - @matz.

As of CRuby2.7 it is only warned. `mruby` prohibits explicitly to
implement the future Ruby3 behavior.

The `debug` build target (`MRB_GC_STRESS` is enabled) on CI have been
compiled but not tested so far. However, I think testing with
`MRB_GC_STRESS` is effective (in fact, I found #4907 bug).

Therefore, I integrated `debug` and `full-debug` build targets to enable
`MRB_GC_STRESS` testing. Testing with `MRB_GC_STRESS` takes a little time,
but compiling takes more time, so CI execution time does not increase due to
decrease of build target.

Enable sanitizer in travis test

Fix potentially use of wrong method cache

#### Example (with `MRB_METHOD_CACHE`)

  ```ruby
  GC.start
  c = Class.new
  p c            #=> #<Class:0x7fd6a180e790>
  c.new          #=> cache `c.new`
  c = nil
  GC.start       #=> `c` is GCed
  r = Range.dup
  p r            #=> #<Class:0x7fd6a180e790>
                 #   [same pointer as `c`]
  r.new(2, 3)    #=> ArgumentError: 'initialize':
                 #   wrong number of arguments (2 for 0)
                 #   [`c.new` is called instead of `r.new`]
  ```

#### Cause

  An entry of method cache is identified by class pointer and method
  id. However, reusing memory after GC may create a class with the same
  pointer as the cached class.

#### Treatment

  Cleared method caches of the class when the class is GCed.

Refine the assertion failure message in mrdb print tests