chainparams: move CCheckpointData into chainparams.h

This unties CChainParams from its dependency on checkpoints. Instead, now it
only depends on the raw checkpoint data.

src/test/Checkpoints_tests.cpp

@@ -22,7 +22,7 @@ BOOST_FIXTURE_TEST_SUITE(Checkpoints_tests, BasicTestingSetup)
 /*
 BOOST_AUTO_TEST_CASE(sanity)
 {
-    const Checkpoints::CCheckpointData& checkpoints = Params(CBaseChainParams::MAIN).Checkpoints();
+    const CCheckpointData& checkpoints = Params(CBaseChainParams::MAIN).Checkpoints();
     BOOST_CHECK(Checkpoints::GetTotalBlocksEstimate(checkpoints) >= 134444);
 }
 */