Consider switching to 'rapid' from 'gopter'

[theunrepentantgeek]

Describe the current behavior

Our code generated gopter tests are valuable, having identified a handful of cases where our code-generated conversion code didn't work correctly, but they take a considerable amount of time to execute.

Describe the improvement

Evaluate use of rapid as an alternative property testing framework. The API claims to be simpler, and performance may be better too.

If the tests run measurably faster, it may be worth switching.