This commit allows lambdas that don't access self to be "made shareable"
regardless of the shareability of self.

For example, the following code used to produce a lambda that was not
shareable:

```ruby
class Foo
  def make_block; lambda { 1234 }; end
end

Ractor.make_shareable(Foo.new.make_block) # exception
```

This lambda was not allowed to be shareable because it could possibly
access `self` which is an unfrozen instance of `Foo`.  However, we know
by looking at the code that it doesn't access the instance of `Foo`, so
I think we should lift this restriction.

Upon calling `make_shareable`, this change scans the instructions of the
block looking for particular instructions that access `self`. If it
sees any of those instructions, then we use the default behavior
(checking sharability of `self`). If we don't see those instructions,
then we'll allow the lambda to be shareable.

For example, this is shareable:

```ruby
def make_block
  foo = 123
  lambda { foo }
end
```

But these are not shareable:

```ruby
def make_block
  lambda { @foo }
end
```

```ruby
def make_block
  lambda { @foo = 1 }
end
```

```ruby
def make_block
  lambda { eval("123") }
end
```

[Feature #21033]