Default reserve size

I would like to suggest adding a user-configurable default reserve size which applies to all future allocations, including types which have not been registered yet.

Motivation: As is, having to explicitly `reserve(theSameValue)` somewhat defeats the polymorphic goal of the library. In most of my use cases, I simply want to set a per-segment initial size and leave it at that.

I like having the option of performing per-segment reserves if I choose, so I think leaving `reserve` as-is is fine, and adding a `setDefaultReserve` would solve the issue.