convert zeroBytesRange and zeroNibblesRange into threadvars

fixes https://github.com/status-im/nim-eth/issues/6

eth/trie/nibbles.nim

@@ -11,8 +11,9 @@ proc initNibbleRange*(bytes: ByteRange): NibblesRange =
   result.ibegin = 0
   result.iend = bytes.len * 2
 
-const
+# can't be a const: https://github.com/status-im/nim-eth/issues/6
-  zeroNibblesRange* = initNibbleRange(zeroBytesRange)
+# we can't initialise it here, but since it's already zeroed memory, we don't need to
+var zeroNibblesRange* {.threadvar.}: NibblesRange
 
 proc `{}`(r: NibblesRange, pos: int): byte {.inline.} =
   ## This is a helper for a more raw access to the nibbles.

eth/trie/trie_defs.nim

@@ -8,8 +8,11 @@ type
   KeccakHash* = MDigest[256]
   BytesContainer* = ByteRange | Bytes | string
 
+# can't be a const: https://github.com/status-im/nim-eth/issues/6
+# we can't initialise it here, but since it's already zeroed memory, we don't need to
+var zeroBytesRange* {.threadvar.}: ByteRange
+
 const
-  zeroBytesRange* = ByteRange()
   blankStringHash* = "c5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470".toDigest
   emptyRlp* = @[128.byte]
   emptyRlpHash* = "56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421".toDigest

tests/trie/test_nibbles.nim

@@ -0,0 +1,9 @@
+import
+  unittest,
+  eth/trie/nibbles
+
+suite "nibbles":
+  test "zeroNibblesRange":
+    # https://github.com/status-im/nim-eth/issues/6
+    check zeroNibblesRange.len == 0
+