feat: split hamt out into separate module, closes #1

Author: achingbrain

package.json

@@ -57,6 +57,7 @@
     "async": "^2.6.1",
     "cids": "~0.5.5",
     "deep-extend": "~0.6.0",
+    "hamt-sharding": "~0.0.1",
     "ipfs-unixfs": "~0.1.16",
     "ipld-dag-pb": "~0.15.0",
     "left-pad": "^1.3.0",
@@ -72,7 +73,6 @@
     "pull-through": "^1.0.18",
     "pull-traverse": "^1.0.3",
     "pull-write": "^1.1.4",
-    "sparse-array": "^1.3.1",
     "stream-to-pull-stream": "^1.7.2"
   },
   "optionalDependencies": {