ocaml-decoders conversion experiment (onivim#1280)

* dependency: add decoders-yojson

* initial conversioon to decoders

* formatting

* add custom obj decoder

* use obj decoder instead of field and monadic bind

* Utility.Json -> JsonEx

* add Json.rei

* add ocaml-decoders to ThirdPartyLicenses.txt

* add license text

ThirdPartyLicenses.txt

@@ -32,6 +32,7 @@ This project incorporates components from the projects listed below. The origina
 25. fmt (https://github.com/dbuenzli/fmt)
 26. LaserWave (https://github.com/Jaredk3nt/laserwave)
 27. camlp5 (https://github.com/camlp5/camlp5)
+28. ocaml-decoders (https://github.com/mattjbray/ocaml-decoders)
 
 %% FiraCode NOTICES AND INFORMATION BEGIN HERE
 ==============================================
@@ -3233,4 +3234,13 @@ END of <template> NOTICES AND INFORMATION
     * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
     * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
-  """
+  """
+
+  - ocaml-decoders is licensed as follows
+  """
+    Copyright 2020 mattjbray
+
+    Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies.
+
+    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+  """