Full implementation of Setup class.

The API (and mostly implementation) closely follows the old C
implementation.
One difference is the handling of HPlus. The user may initialize it
through UpdateHPlus(Term::Factory) when she is done adding clauses to
the setup.

The code is untested!

Lazy copies of sets would perhaps be helpful.

Also replaced a few unsigned integers with normal int (in adherance to
C++ style guide). The IDs of normal StdNames are limited to
non-negative integers; negative integers are used for placeholder names.