dynamic NaN-safe mode switching #181

Follow-up issue to #179. From my comment there:

In a future PR, we could add AbstractConfig constructors like GradientConfig(x, Chunk{N}, NaNSafe{true}). The generated dual numbers would then contain partials of type Partials{N,T,true}, which would dispatch to the NaN-safe versions of methods. Downstream modules could then support passing that option through their APIs (or simply setting it to a reasonable default for themselves internally).