Ruby

Initialize the Prism::Source directly with all 3 fields for the C extension

• Faster that way:
  $ ruby -Ilib -rprism -rbenchmark/ips -e 'Benchmark.ips { |x| x.report("parse") { Prism.parse("1 + 2") } }'
  195.722k (± 0.5%) i/s
  rb_iv_set():
  179.609k (± 0.5%) i/s
  rb_funcall():
  190.030k (± 0.3%) i/s
  before this PR:
  183.319k (± 0.4%) i/s