Separate public-facing and internal-facing interfaces to Statistics.

Separate public-facing and internal-facing interfaces to Statistics.

The external interface (e.g., what's answered by ExprManager::getStatistics() and SmtEngine::getStatistics()) is a snapshot of the current statistics (rather than a reference to the actual StatisticsRegistry).

The StatisticsRegistry is now internal-only.  However, it's built as a convenience library so that the parser and driver can use it too (by re-linking against it).

This is part of the ongoing effort to clean up the public interface.

(this commit was certified error- and warning-free by the test-and-commit script.)