Use JSON as submodule

EOM

.gitmodules

@@ -10,3 +10,7 @@
 	path = ext/fmt
 	url = https://github.com/fmtlib/fmt.git
 	branch = master
+[submodule "ext/json"]
+	path = ext/json
+	url = https://github.com/nlohmann/json.git
+	branch = master

json @ db78ac1d

+Subproject commit db78ac1d7716f56fc9f1b030b715f872f93964e4

ext/json/LICENSE.MIT

-MIT License 
-
-Copyright (c) 2013-2020 Niels Lohmann
-
-Permission is hereby granted, free of charge, to any person obtaining a copy
-of this software and associated documentation files (the "Software"), to deal
-in the Software without restriction, including without limitation the rights
-to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-copies of the Software, and to permit persons to whom the Software is
-furnished to do so, subject to the following conditions:
-
-The above copyright notice and this permission notice shall be included in all
-copies or substantial portions of the Software.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
-FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
-AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
-LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
-OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-SOFTWARE.