refactor: Replace all uses of boost::optional with our own Optional type

After this:

- `boost::optional` is no longer used directly (only through `Optional`
    which is an alias for it)
- `boost/optional.hpp` is only included in one place

src/psbt.h

@@ -387,7 +387,7 @@ struct PSBTOutput
 /** A version of CTransaction with the PSBT format*/
 struct PartiallySignedTransaction
 {
-    boost::optional<CMutableTransaction> tx;
+    Optional<CMutableTransaction> tx;
     std::vector<PSBTInput> inputs;
     std::vector<PSBTOutput> outputs;
     std::map<std::vector<unsigned char>, std::vector<unsigned char>> unknown;

src/txmempool.cpp

@@ -8,6 +8,7 @@
 #include <consensus/consensus.h>
 #include <consensus/tx_verify.h>
 #include <consensus/validation.h>
+#include <optional.h>
 #include <validation.h>
 #include <policy/policy.h>
 #include <policy/fees.h>
@@ -155,7 +156,7 @@ bool CTxMemPool::CalculateMemPoolAncestors(const CTxMemPoolEntry &entry, setEntr
         // GetMemPoolParents() is only valid for entries in the mempool, so we
         // iterate mapTx to find parents.
         for (unsigned int i = 0; i < tx.vin.size(); i++) {
-            boost::optional<txiter> piter = GetIter(tx.vin[i].prevout.hash);
+            Optional<txiter> piter = GetIter(tx.vin[i].prevout.hash);
             if (piter) {
                 parentHashes.insert(*piter);
                 if (parentHashes.size() + 1 > limitAncestorCount) {
@@ -860,11 +861,11 @@ const CTransaction* CTxMemPool::GetConflictTx(const COutPoint& prevout) const
     return it == mapNextTx.end() ? nullptr : it->second;
 }
 
-boost::optional<CTxMemPool::txiter> CTxMemPool::GetIter(const uint256& txid) const
+Optional<CTxMemPool::txiter> CTxMemPool::GetIter(const uint256& txid) const
 {
     auto it = mapTx.find(txid);
     if (it != mapTx.end()) return it;
-    return boost::optional<txiter>{};
+    return Optional<txiter>{};
 }
 
 CTxMemPool::setEntries CTxMemPool::GetIterSet(const std::set<uint256>& hashes) const

src/txmempool.h

@@ -17,6 +17,7 @@
 #include <coins.h>
 #include <crypto/siphash.h>
 #include <indirectmap.h>
+#include <optional.h>
 #include <policy/feerate.h>
 #include <primitives/transaction.h>
 #include <sync.h>
@@ -602,7 +603,7 @@ public:
     const CTransaction* GetConflictTx(const COutPoint& prevout) const EXCLUSIVE_LOCKS_REQUIRED(cs);
 
     /** Returns an iterator to the given hash, if found */
-    boost::optional<txiter> GetIter(const uint256& txid) const EXCLUSIVE_LOCKS_REQUIRED(cs);
+    Optional<txiter> GetIter(const uint256& txid) const EXCLUSIVE_LOCKS_REQUIRED(cs);
 
     /** Translate a set of hashes into a set of pool iterators to avoid repeated lookups */
     setEntries GetIterSet(const std::set<uint256>& hashes) const EXCLUSIVE_LOCKS_REQUIRED(cs);

src/wallet/coincontrol.h

@@ -5,13 +5,12 @@
 #ifndef BITCOIN_WALLET_COINCONTROL_H
 #define BITCOIN_WALLET_COINCONTROL_H
 
+#include <optional.h>
 #include <policy/feerate.h>
 #include <policy/fees.h>
 #include <primitives/transaction.h>
 #include <wallet/wallet.h>
 
-#include <boost/optional.hpp>
-
 const int DEFAULT_MIN_DEPTH = 0;
 const int DEFAULT_MAX_DEPTH = 9999999;
 
@@ -22,7 +21,7 @@ public:
     //! Custom change destination, if not set an address is generated
     CTxDestination destChange;
     //! Override the default change type if set, ignored if destChange is set
-    boost::optional<OutputType> m_change_type;
+    Optional<OutputType> m_change_type;
     //! If false, allows unselected inputs, but requires all selected inputs be used
     bool fAllowOtherInputs;
     //! Includes watch only addresses which are solvable
@@ -30,11 +29,11 @@ public:
     //! Override automatic min/max checks on fee, m_feerate must be set if true
     bool fOverrideFeeRate;
     //! Override the wallet's m_pay_tx_fee if set
-    boost::optional<CFeeRate> m_feerate;
+    Optional<CFeeRate> m_feerate;
     //! Override the default confirmation target if set
-    boost::optional<unsigned int> m_confirm_target;
+    Optional<unsigned int> m_confirm_target;
     //! Override the wallet's m_signal_rbf if set
-    boost::optional<bool> m_signal_bip125_rbf;
+    Optional<bool> m_signal_bip125_rbf;
     //! Avoid partial use of funds sent to a given address
     bool m_avoid_partial_spends;
     //! Forbids inclusion of dirty (previously used) addresses

src/wallet/coinselection.cpp

@@ -4,11 +4,10 @@
 
 #include <wallet/coinselection.h>
 
+#include <optional.h>
 #include <util/system.h>
 #include <util/moneystr.h>
 
-#include <boost/optional.hpp>
-
 // Descending order comparator
 struct {
     bool operator()(const OutputGroup& a, const OutputGroup& b) const
@@ -219,7 +218,7 @@ bool KnapsackSolver(const CAmount& nTargetValue, std::vector<OutputGroup>& group
     nValueRet = 0;
 
     // List of values less than target
-    boost::optional<OutputGroup> lowest_larger;
+    Optional<OutputGroup> lowest_larger;
     std::vector<OutputGroup> applicable_groups;
     CAmount nTotalLower = 0;