Make the Share generator generate smaller shares more often

Instead of distributing the range of the size of most fields of the share linearly over some range, use an exponential distribution so that values in the result are exponentially more likely to be small. Large values are still possible, just less likely.

Since most encoding/decoding bugs are possible with even the smallest allowed amount of data this seems like a reasonable tradeoff for better test suite performance.

Before the change "Share round-trips through put / get" takes 165 seconds on my laptop. After the change it takes 26 seconds.