feat(tvix/eval): Implement builtins.fromJSON

Using `serde_json` for parsing JSON here, plus an `impl FromJSON for
Value`. The latter is primarily to stay "dependency light" for now -
likely going with an actual serde `Deserialize` impl in the future is
going to be way better as it allows saving significantly on intermediary
allocations.

Change-Id: I152a0448ff7c87cf7ebaac927c38912b99de1c18
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6920
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.exp

@@ -0,0 +1 @@
+"quote \" reverse solidus \\ solidus / backspace  formfeed  newline \n carriage return \r horizontal tab \t 1 char unicode encoded backspace  1 char unicode encoded e with accent é 2 char unicode encoded s with caron š 3 char unicode encoded rightwards arrow →"

tvix/eval/src/tests/tvix_tests/eval-okay-fromjson-escapes.nix

@@ -0,0 +1,3 @@
+# This string contains all supported escapes in a JSON string, per json.org
+# \b and \f are not supported by Nix
+builtins.fromJSON ''"quote \" reverse solidus \\ solidus \/ backspace \b formfeed \f newline \n carriage return \r horizontal tab \t 1 char unicode encoded backspace \u0008 1 char unicode encoded e with accent \u00e9 2 char unicode encoded s with caron \u0161 3 char unicode encoded rightwards arrow \u2192"''

tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.exp

@@ -0,0 +1 @@
+[ { Image = { Animated = false; Height = 600; IDs = [ 116 943 234 38793 true false null -100 ]; Latitude = 37.7668; Longitude = -122.3959; Thumbnail = { Height = 125; Url = "http://www.example.com/image/481989943"; Width = 100; }; Title = "View from 15th Floor"; Width = 800; }; } { name = "a"; value = "b"; } ]

tvix/eval/src/tests/tvix_tests/eval-okay-fromjson.nix

@@ -0,0 +1,23 @@
+[
+# RFC 7159, section 13.
+  (builtins.fromJSON
+    ''
+    {
+      "Image": {
+          "Width":  800,
+          "Height": 600,
+          "Title":  "View from 15th Floor",
+          "Thumbnail": {
+              "Url":    "http://www.example.com/image/481989943",
+              "Height": 125,
+              "Width":  100
+          },
+          "Animated" : false,
+          "IDs": [116, 943, 234, 38793, true  ,false,null, -100],
+          "Latitude":  37.7668,
+          "Longitude": -122.3959
+      }
+    }
+  '')
+  (builtins.fromJSON ''{"name": "a", "value": "b"}'')
+]