consensus: From Heartwood activation, use Rust Equihash validator

The C++ and Rust Equihash validators are intended to have an identical
set of valid Equihash solutions, so this should merely be an
implementation detail. However, deploying the Rust validator at the same
time as a network upgrade reduces the risk of an unintentional consensus
divergence due to undocumented behaviour in either implementation.

Once Heartwood has activated on mainnet, we can verify that all
pre-Heartwood blocks satisfy the Rust validator, and then remove the C++
validator and make Equihash-checking non-contextual again.

src/pow.cpp

@@ -13,6 +13,7 @@
 #include "streams.h"
 #include "uint256.h"
 
+#include <librustzcash.h>
 #include "sodium.h"
 
 unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
@@ -106,6 +107,17 @@ bool CheckEquihashSolution(const CBlockHeader *pblock, int nHeight, const Consen
     // I||V
     CDataStream ss(SER_NETWORK, PROTOCOL_VERSION);
     ss << I;
+
+    // From Heartwood activation, check with the Rust validator
+    if (params.NetworkUpgradeActive(nHeight, Consensus::UPGRADE_HEARTWOOD)) {
+        return librustzcash_eh_isvalid(
+            n, k,
+            (unsigned char*)&ss[0], ss.size(),
+            pblock->nNonce.begin(), pblock->nNonce.size(),
+            pblock->nSolution.data(), pblock->nSolution.size());
+    }
+
+    // Before Heartwood activation, check with the C++ validator
     ss << pblock->nNonce;
 
     // H(I||V||...