Another pass: rough shape of tree and tests

It isn't working yet, the tests don't pass.

But it is on the way!

test/conftest.py

@@ -0,0 +1,22 @@
+import hashlib
+
+import pytest
+
+
+@pytest.fixture
+def leaf():
+    def _leaf(node):
+        return hashlib.sha256(leaf.data).hexdigest()
+
+    return _leaf
+
+
+@pytest.fixture
+def parent():
+    def _parent(left, right):
+        sha256 = hashlib.sha256()
+        sha256.update(left)
+        sha256.update(right)
+        return sha256.hexdigest()
+
+    return _parent