[refactor] Add deploymentstatus.h

Provides DeploymentEnabled, DeploymentActiveAt, and DeploymentActiveAfter
helpers for checking the status of buried deployments. Can be overloaded
so the same syntax works for non-buried deployments, allowing future
soft forks to be changed from signalled to buried deployments without
having to touch the implementation code.

Replaces IsWitnessEnabled and IsScriptWitnessEnabled.

src/deploymentstatus.h

@@ -0,0 +1,33 @@
+// Copyright (c) 2020 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_DEPLOYMENTSTATUS_H
+#define BITCOIN_DEPLOYMENTSTATUS_H
+
+#include <chain.h>
+
+#include <limits>
+
+/** Determine if a deployment is active for the next block */
+inline bool DeploymentActiveAfter(const CBlockIndex* pindexPrev, const Consensus::Params& params, Consensus::BuriedDeployment dep)
+{
+    assert(Consensus::ValidDeployment(dep));
+    return (pindexPrev == nullptr ? 0 : pindexPrev->nHeight + 1) >= params.DeploymentHeight(dep);
+}
+
+/** Determine if a deployment is active for this block */
+inline bool DeploymentActiveAt(const CBlockIndex& index, const Consensus::Params& params, Consensus::BuriedDeployment dep)
+{
+    assert(Consensus::ValidDeployment(dep));
+    return index.nHeight >= params.DeploymentHeight(dep);
+}
+
+/** Determine if a deployment is enabled (can ever be active) */
+inline bool DeploymentEnabled(const Consensus::Params& params, Consensus::BuriedDeployment dep)
+{
+    assert(Consensus::ValidDeployment(dep));
+    return params.DeploymentHeight(dep) != std::numeric_limits<int>::max();
+}
+
+#endif // BITCOIN_DEPLOYMENTSTATUS_H