This change fixes the bug described in https://bugs.ruby-lang.org/issues/17880.

Checking `ractor_shareable_p` will cause the method to call back into
Ruby. Anything calling this method can't be a leaf instruction,
otherwise it could crash. By adding `attr bool leaf = false` we no
longer crash because it marks the function as not a leaf.

Here's a simplified reproduction script:

```ruby
require "set"

class Id
  attr_reader :db_id
  def initialize(db_id)
    @db_id = db_id
  end

  def ==(other)
    other.class == self.class && other.db_id == db_id
  end
  alias_method :eql?, :==

  def hash
    10
  end

  def <=>(other)
    db_id <=> other.db_id if other.is_a?(self.class)
  end
end

class Namespace
  IDS = Set[
    Id.new(1).freeze,
    Id.new(2).freeze,
    Id.new(3).freeze,
    Id.new(4).freeze,
  ].freeze

  class << self
    def test?(id)
      IDS.include?(id)
    end
  end
end

p Namespace.test?(Id.new(1))
p Namespace.test?(Id.new(5))
```

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>