Mark ruby bindings as ractor safe (#1283)

Mark ruby bindings as ractor safe (#1283)

Ruby has a new method for concurrency called Ractors.

For C bindings to be used inside a Ractor they need to be marked
Ractor-safe [1]:
  If an extension desires to be marked as Ractor-safe the extension
  should call rb_ext_ractor_safe(true) at the Init_ function for the
  extension, and all defined methods will be marked as Ractor-safe.

By marking them Ractor-safe ruby programs can generate graphs concurrently.

See also ruby/ruby#3824 for more information concerning this method.

[1] https://docs.ruby-lang.org/en/master/extension_rdoc.html#label-Appendix+F.+Ractor+support