Fix broken constant parameter blocks

This fixes the bug introduced in a previous commit,
and adds a test to check that constant parameter
blocks work as expected.

This also refactors the Solver/SolverImpl split so
that SolverImpl is no longer a friend of Problem;
instead, Solver is. This makes it possible to
verify the invariant on parameter block states in
the unit test, and is a more symmetric design
anyway.

Bug: 51
Change-Id: Id503f5b526cfb8bc24aae3aaad2e414b14063d78

include/ceres/problem.h

@@ -50,13 +50,13 @@ namespace ceres {
 class CostFunction;
 class LossFunction;
 class LocalParameterization;
+class Solver;
 
 namespace internal {
 class Preprocessor;
 class ProblemImpl;
 class ParameterBlock;
 class ResidualBlock;
-class SolverImpl;
 }  // namespace internal
 
 // A ResidualBlockId is a handle clients can use to delete residual
@@ -255,7 +255,7 @@ class Problem {
   int NumResiduals() const;
 
  private:
-  friend class internal::SolverImpl;
+  friend class Solver;
   internal::scoped_ptr<internal::ProblemImpl> problem_impl_;
   CERES_DISALLOW_COPY_AND_ASSIGN(Problem);
 };