Reduce builder copies using a managed reference

[mtdowling]

This commit reduces the number of copies builders need to make when
building up immutable objects.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mtdowling]

Relevant benchmarks:

Before:

Benchmark                                  Mode  Cnt   Score   Error  Units
Selectors.loadsIdlModelWithoutValidation   avgt    3  57.376 ± 5.336  us/op
Selectors.loadsJsonModelWithoutValidation  avgt    3  73.979 ± 9.156  us/op

After:

Benchmark                                  Mode  Cnt   Score    Error  Units
Selectors.loadsIdlModelWithoutValidation   avgt    3  50.852 ± 11.004  us/op
Selectors.loadsJsonModelWithoutValidation  avgt    3  61.740 ±  6.504  us/op