This change reduces the recursion level, but does not solve the stack
overflow issue entirely.

But #3421 still cause stack overflow error due to infinite recursion.
To prevent overflow, we need to add different stack depth check.

The issue (and the fix) was reported by https://hackerone.com/dgaletic

This issue was reported by https://hackerone.com/d4nny

The issue was reported by https://hackerone.com/titanous

Fixed aspec for Kernel#caller

Revert "Optimization for String#* for 1-byte strings"

There's incompatibility left for mruby.

When you return from `ensure` clause, mruby simply ignores the return
value.  CRuby returns from the method squashing the exception raised.

```
def f
  no_such_method() # NoMethodError
ensure
  return 22
end

p f() # CRuby prints `22`
```

This reverts commit d1bc7cae.

This issue was reported by https://hackerone.com/aerodudrizzt

OP_RETURN accesses ci[1]->stackent that might be broken; fix #3442

remove skip that shouldn't be necessary anymore.

the test should pass after https://github.com/mruby/mruby/commit/8f4a929e1a01c8d6176fb53a9ef5dff6de632959.

Otherwise String#size may return wrong length; fix #3448

String#ljust and String#rjust reimplemented with optimized Ruby

Fix build error when trace mode

String#ljust and String#rjust reimplementation (fix #3445)

 - String#ljust and String#rjust are now C functions to improve performance
 - infinite loop because of an empty padding argument is now prevented (ArgumentError is raised)
 - extra tests for ljust/rjust added

Rakefile

```
file "a.txt" do
  system "echo hello > a.txt"
end

file "b.txt" => "a.txt" do
  system "cp a.txt b.txt"
end

task :default => "b.txt"
```

```
$ touch b.txt
$ ruby minirake --trace
Invoke default (already=[], needed=[true])
rake aborted!
No such file or directory @ rb_file_s_stat - a.txt
minirake:241:in `stat'
minirake:241:in `timestamp'
minirake:233:in `block in needed?'
minirake:233:in `collect'
minirake:233:in `needed?'
minirake:91:in `invoke'
minirake:95:in `block in invoke'
minirake:95:in `each'
minirake:95:in `invoke'
minirake:467:in `block in run'
minirake:466:in `each'
minirake:466:in `run'
minirake:484:in `<main>'
```

in mrbgems/mruby-bin-debugger.

According to the valgrind log attached to #3438, proc->body.irep
may be NULL in some cases.

Maybe related to #3438

Dinko Galetic and Denis Kasak reported the issue and the fix.
(via https://hackerone.com/dgaletic).

Fix interpolation escaping in String.inspect

Kernel#local_variables: Make result array unique

http://hkdnet.hatenablog.com/entry/2017/02/06/080000 (Japanese)

Now `eval()` can call Fiber.yield etc.

Calling `mrb_funcall()` and `mrb_yield()` (and their related
functions) are discouraged unless absolutely necessary, because
it can cause this kind of issues very easily.