Make sure JSON macro performance is reasonable

[gmethvin]

The way we construct the Writes implementations with the JSON macros is not particularly efficient, as I have noticed in a few real-world apps. We should devise some tests to determine how much a performance impact there is versus manually constructing the object, and see where we can improve.

[marcospereira]

I wonder if it is better to have a test here or in prune.

Maybe prune is a better place since we can track the performance evolution in a scenario more close to the real apps.

What do you think?

[gmethvin]

We can add tests to prune but I think microbenchmarks are the best idea to track perf of individual features.