OpenapiFirst.load requires a lot of memory

[ahx]

Benchmark.memory do |x|
  x.report { OpenapiFirst.load('../spec/data/large.yaml') }
end

26.563M memsize (     4.364M retained)
268.592k objects (    38.065k retained)
50.000  strings (    50.000  retained)

I see several ways to address this:

1. Reduce overall memory usage
2. Reduce memory usage while parsing the OAD

Both are a good option, I think.