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

internal/ceres/program.cc

@@ -90,7 +90,7 @@ void Program::CopyParameterBlockStateToUserState() {
   }
 }
 
-bool Program::CopyUserStateToParameterBlocks() {
+bool Program::SetParameterBlockStatePtrsToUserStatePtrs() {
   for (int i = 0; i < parameter_blocks_.size(); ++i) {
     if (!parameter_blocks_[i]->SetState(parameter_blocks_[i]->user_state())) {
       return false;