move-only: Create src/kernel/mempool_removal_reason.h

This is needed for a future commit. Can be reviewed with:
--color-moved=dimmed-zebra

src/kernel/mempool_removal_reason.cpp

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+// Copyright (c) 2016-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit/.
+
+#include <kernel/mempool_removal_reason.h>
+
+#include <cassert>
+#include <string>
+
+std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept
+{
+    switch (r) {
+        case MemPoolRemovalReason::EXPIRY: return "expiry";
+        case MemPoolRemovalReason::SIZELIMIT: return "sizelimit";
+        case MemPoolRemovalReason::REORG: return "reorg";
+        case MemPoolRemovalReason::BLOCK: return "block";
+        case MemPoolRemovalReason::CONFLICT: return "conflict";
+        case MemPoolRemovalReason::REPLACED: return "replaced";
+    }
+    assert(false);
+}

src/kernel/mempool_removal_reason.h

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+// Copyright (c) 2016-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or https://opensource.org/license/mit/.
+
+#ifndef BITCOIN_KERNEL_MEMPOOL_REMOVAL_REASON_H
+#define BITCOIN_KERNEL_MEMPOOL_REMOVAL_REASON_H
+
+#include <string>
+
+/** Reason why a transaction was removed from the mempool,
+ * this is passed to the notification signal.
+ */
+enum class MemPoolRemovalReason {
+    EXPIRY,      //!< Expired from mempool
+    SIZELIMIT,   //!< Removed in size limiting
+    REORG,       //!< Removed for reorganization
+    BLOCK,       //!< Removed for block
+    CONFLICT,    //!< Removed for conflict with in-block transaction
+    REPLACED,    //!< Removed for replacement
+};
+
+std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept;
+
+#endif // BITCOIN_KERNEL_MEMPOOL_REMOVAL_REASON_H