Implementation of simple STL-like forward iterators on Array and List.

This provides some simple STL-like forward iterator classes, both
const and non-const, for the Array and List classes.

It is recommended that new code use the STL-like features of these
classes, and that existing code be retrofitted as appropriate.
Additional STL-compatible operations on these classes should be
added as required.
Bug fix in PureStrategyProfileRep::GetBestResponse; convert to iterators

* Fixes a bug in PureStrategyProfileRep::GetBestResponse, in which the
  maximal payoff was not being updated correctly.
* Rewrites the recent contributions determining Nash equilibrium and
  best responses to use the iterator idiom over strategy sets of
  the player.
Replace GamePlayerIterator with a standard iterator on an Array of player rep.
Remove GameStrategyIterator in favor of more standard STL-like iterator.
Replace SupportStrategyIterator with more standard iterator on Array.
Implement overload of [] to get strategy from mixed profile.

This implements operator [] on a mixed strategy profile, when called
with a player as argument.  The return value is the vector of
probabilities played over the player's strategies.

This is now used in gambit-enummixed to extract the individual
players' strategies, refactoring the code that was previously in
that location.