Move crates under a sub folder to clean up the code

crates/flatten-serde-json/README.md

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+# Flatten serde Json
+
+This crate flatten [`serde_json`](https://docs.rs/serde_json/latest/serde_json/) `Object` in a format
+similar to [elastic search](https://www.elastic.co/guide/en/elasticsearch/reference/current/nested.html).
+
+## Examples
+
+### There is nothing to do
+
+```json
+{
+  "id": "287947",
+  "title": "Shazam!",
+  "release_date": 1553299200,
+  "genres": [
+    "Action",
+    "Comedy",
+    "Fantasy"
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "id": "287947",
+  "title": "Shazam!",
+  "release_date": 1553299200,
+  "genres": [
+    "Action",
+    "Comedy",
+    "Fantasy"
+  ]
+}
+```
+
+------------
+
+### Objects
+
+```json
+{
+  "a": {
+    "b": "c",
+    "d": "e",
+    "f": "g"
+  }
+}
+```
+
+Flattens to:
+```json
+{
+  "a.b": "c",
+  "a.d": "e",
+  "a.f": "g"
+}
+```
+
+------------
+
+### Array of objects
+
+```json
+{
+  "a": [
+    { "b": "c" },
+    { "b": "d" },
+    { "b": "e" },
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "a.b": ["c", "d", "e"],
+}
+```
+
+------------
+
+### Array of objects with normal value in the array
+
+```json
+{
+  "a": [
+    42,
+    { "b": "c" },
+    { "b": "d" },
+    { "b": "e" },
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "a": 42,
+  "a.b": ["c", "d", "e"],
+}
+```
+
+------------
+
+### Array of objects of array of objects of ...
+
+```json
+{
+  "a": [
+    "b",
+    ["c", "d"],
+    { "e": ["f", "g"] },
+    [
+        { "h": "i" },
+        { "e": ["j", { "z": "y" }] },
+    ],
+    ["l"],
+    "m",
+  ]
+}
+```
+
+Flattens to:
+```json
+{
+  "a": ["b", "c", "d", "l", "m"],
+  "a.e": ["f", "g", "j"],
+  "a.h": "i",
+  "a.e.z": "y",
+}
+```
+
+------------
+
+### Collision between a generated field name and an already existing field
+
+```json
+{
+  "a": {
+    "b": "c",
+  },
+  "a.b": "d",
+}
+```
+
+Flattens to:
+```json
+{
+  "a.b": ["c", "d"],
+}
+```
+