dvc: introduce local build cache (#3603)

This patch introduces `.dvc/cache/stages` that is used to store previous
runs and their results, which could then be reused later when we stumble
upon the same command with the same deps and outs.

Format of build cache entries is single-line json, which is readable by
humans and might also be used for lock files discussed in #1871.

Related to #1871
Local part of #1234

tests/unit/utils/test_utils.py

@@ -6,6 +6,7 @@ import pytest
 from dvc.path_info import PathInfo
 from dvc.utils import (
     file_md5,
+    dict_sha256,
     resolve_output,
     fix_env,
     relpath,
@@ -155,3 +156,28 @@ def test_hint_on_lockfile():
     with pytest.raises(Exception) as exc:
         assert parse_target("pipelines.lock:name")
     assert "pipelines.yaml:name" in str(exc.value)
+
+
+@pytest.mark.parametrize(
+    "d,sha",
+    [
+        (
+            {
+                "cmd": "echo content > out",
+                "deps": {"dep": "2254342becceafbd04538e0a38696791"},
+                "outs": {"out": "f75b8179e4bbe7e2b4a074dcef62de95"},
+            },
+            "f472eda60f09660a4750e8b3208cf90b3a3b24e5f42e0371d829710e9464d74a",
+        ),
+        (
+            {
+                "cmd": "echo content > out",
+                "deps": {"dep": "2254342becceafbd04538e0a38696791"},
+                "outs": ["out"],
+            },
+            "a239b67073bd58affcdb81fff3305d1726c6e7f9c86f3d4fca0e92e8147dc7b0",
+        ),
+    ],
+)
+def test_dict_sha256(d, sha):
+    assert dict_sha256(d) == sha