Use sorted RLP serialisation for LRU cache

why:
  previously, table data were stored with the table iterator. while
  loading a table with permuted entries will always reconstruct equivalent
  tables (in the sense of `==`), serialisation data are not comparable.

  this patch produces always the same serialised data for equivalent
  tables.

nimbus/utils/lru_cache.nim

@@ -159,8 +159,16 @@ proc append*[K,V](rw: var RlpWriter; data: LruData[K,V]) {.inline.} =
   rw.append(data.first)
   rw.append(data.last)
   rw.startList(data.tab.len)
-  for key,value in data.tab.pairs:
+  # store keys in LRU order
-    rw.append((key, value))
+  if 0 < data.tab.len:
+    var key = data.first
+    for _ in 0 ..< data.tab.len - 1:
+      var value = data.tab[key]
+      rw.append((key, value))
+      key = value.nxt
+    rw.append((key, data.tab[key]))
+    if key != data.last:
+      raiseAssert "Garbled LRU cache next/prv references"
 
 proc read*[K,V](rlp: var Rlp; Q: type LruData[K,V]): Q {.inline.} =
   ## Generic support for `rlp.decode(bytes)` for loading the data part

tests/test_lru_cache.nim

@@ -157,6 +157,35 @@ proc doSerialiserTest(noisy: bool) =
 
   doAssert s1 == rlp.encode(c2.data)
 
+proc doSerialiseSingleEntry(noisy: bool) =
+
+  proc say(a: varargs[string]) =
+    say(noisy = noisy, args = a)
+
+  var
+    c1 = createTestCache()
+    value = c1.getLruItem(77)
+    queue = c1.toKeyList
+    values = c1.toValueList
+
+  say &"c1: append {value} => {queue}"
+
+  var
+    s1 = rlp.encode(c1.data)
+    c2 = createTestCache()
+
+  say &"serialised[{s1.len}]: {s1}"
+
+  c2.clearLruCache
+  doAssert c1 != c2
+
+  c2.data = s1.decode(type c2.data)
+  doAssert c1 == c2
+
+  say &"c2Specs: {c2.maxItems} {c2.first} {c2.last} ..."
+
+  doAssert s1 == rlp.encode(c2.data)
+
 
 proc lruCacheMain*(noisy = defined(debug)) =
   suite "LRU Cache":
@@ -167,6 +196,10 @@ proc lruCacheMain*(noisy = defined(debug)) =
     test "Rlp Serialise & Load":
       doSerialiserTest(noisy)
 
+    test "Rlp Single Entry Test":
+      doSerialiseSingleEntry(noisy)
+
+
 when isMainModule:
   lruCacheMain()