Exponential policy for new pool size

In response to https://github.com/D-Programming-Language/druntime/pull/817#discussion_r13396221

My benchmarks for the samplings were obviously a failure due to the high overhead, weighting is necessary but each bin has to be assessed for its pointers' weight during collection. Resulted in a 250-300% worsening on my implementation.

This change properly settles the question of too many collections.