Allow lambdas that don't access self to be made shareable

[tenderlove]

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:

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:

def make_block
  foo = 123
  lambda { foo }
end

But these are not shareable:

def make_block
  lambda { @foo }
end

def make_block
  lambda { @foo = 1 }
end

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

[Feature #21033]