There is no such thing:

```rb
irb(main):001> sprintf("%F", 123)
(irb):1:in 'Kernel#sprintf': malformed format string - %F (ArgumentError)
```

This change separates the master box from the root box, as the single
master copy of boxes.
Before this change, the root box is the source of copies, and also
it runs builtin classes' code. Builtin code makes changes by
requiring files, thus the source of copies is mutable, be different
from the past one. This causes different internal states of boxes,
depending on when it is created.
After this change, all boxes are created from the master box, and
the master box never runs code, so it realizes the immutable source
of copies.

This also makes is possible to separate RubyGems (and other default
gems) from each boxes. All boxes will have their own copies of RubyGems.
RubyGems has its internal state, but it'll be separated between boxes
after this change.

As well as `putchilledstring` as `dupchilledstring`.

This is more consistent with similar `duparray` and `duphash`
instructions and better reflect it's behavior.

[Feature #21796] unpack variant `^` that returns the current offset

* Revert "Revert pack/unpack support for LEB128"

This reverts commit 77c3a9e447ec477be39e00072e1ce3348d0f4533.

* Update specs for LEB128