scripted-diff: Move src/kernel/coinstats to kernel::

Introduces a new kernel:: namespace and move all of src/kernel/coinstats
under it.

In the verify script, lines like:

line="$(grep -n 'namespace node {' -- src/kernel/coinstats.h | tail -n1 | cut -d: -f1)"
sed -i -e "${line}s@namespace node {@namespace kernel {@" -- src/kernel/coinstats.h

Are intended to replace only the last instance of "namespace node" with
"namespace kernel", this is to avoid replacing forward declarations of
things inside the node:: namespace.

-BEGIN VERIFY SCRIPT-
sed -E -i 's@namespace node@namespace kernel@g' -- src/kernel/coinstats.cpp

line="$(grep -n 'namespace node {' -- src/kernel/coinstats.h | tail -n1 | cut -d: -f1)"
sed -i -e "${line}s@namespace node {@namespace kernel {@" -- src/kernel/coinstats.h

line="$(grep -n '// namespace node' -- src/kernel/coinstats.h | tail -n1 | cut -d: -f1)"
sed -i -e "${line}s@// namespace node@// namespace kernel@" -- src/kernel/coinstats.h

things='(CCoinsStats|CoinStatsHashType|GetBogoSize|TxOutSer|ComputeUTXOStats)'
git grep -lE 'node::'"$things" | xargs sed -E -i 's@node::'"$things"'@kernel::\1@g'
sed -E -i 's@'"$things"'@kernel::\1@g' -- src/node/coinstats.cpp src/node/coinstats.h
sed -E -i 's@BlockManager@node::\0@g' -- src/kernel/coinstats.cpp
-END VERIFY SCRIPT-

src/kernel/coinstats.cpp

@@ -15,7 +15,7 @@
 
 #include <map>
 
-namespace node {
+namespace kernel {
 
 CCoinsStats::CCoinsStats(int block_height, const uint256& block_hash)
     : nHeight(block_height),
@@ -135,7 +135,7 @@ static bool ComputeUTXOStats(CCoinsView* view, CCoinsStats& stats, T hash_obj, c
     return true;
 }
 
-std::optional<CCoinsStats> ComputeUTXOStats(CoinStatsHashType hash_type, CCoinsView* view, BlockManager& blockman, const std::function<void()>& interruption_point)
+std::optional<CCoinsStats> ComputeUTXOStats(CoinStatsHashType hash_type, CCoinsView* view, node::BlockManager& blockman, const std::function<void()>& interruption_point)
 {
     CBlockIndex* pindex = WITH_LOCK(::cs_main, return blockman.LookupBlockIndex(view->GetBestBlock()));
     CCoinsStats stats{Assert(pindex)->nHeight, pindex->GetBlockHash()};
@@ -184,4 +184,4 @@ static void FinalizeHash(MuHash3072& muhash, CCoinsStats& stats)
 }
 static void FinalizeHash(std::nullptr_t, CCoinsStats& stats) {}
 
-} // namespace node
+} // namespace kernel