Fix for string reference corruption issue

[ksss]

If GC.compact occurs after parsing, strings referenced from the parse result are corrupted. Below is a reproduction script.
In this case, it is intentionally reproducing a scenario where GC.compact occurs during parsing.

require 'rbs'

module Patch
  def to_i(...)
    super(...)
  end
end

String.prepend(Patch)

TracePoint.new(:call) do |tp|
  GC.start
  GC.start
  GC.compact
end.enable(target: String.instance_method(:to_i))

errors = 0
n = 100
n.times do |i|
  1000.times { |i| "\x00" * (1000); "X#{i}" * 100 }
  len = 500
  var = ("A" * len + "_V#{i}").to_sym
  type = "(42 | #{var}) -> Array[#{var}]"

  result = RBS::Parser.parse_method_type(type, variables: [var])
  if var == result.type.return_type.args.first.name
    print "."
  else
    errors += 1
    print "X"
  end
end

puts "\n#{errors}/n errors (#{(errors * 100.0 / n).round(1)}%)"
exit(errors > 0 ? 1 : 0)

To fix this issue, similar to #2822 , strings of symbols given as arguments are copied and retained. If it is an owned type, free() will also be performed when the constant pool is freed.