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/builtins/mod.rs

@@ -271,6 +271,11 @@ fn pure_builtins() -> Vec<Builtin> {
                 Ok(res)
             },
         ),
+        Builtin::new("fromJSON", &[true], |args: Vec<Value>, _: &mut VM| {
+            let json_str = args[0].to_str()?;
+            let json: serde_json::Value = serde_json::from_str(&json_str)?;
+            json.try_into()
+        }),
         Builtin::new("genList", &[true, true], |args: Vec<Value>, vm: &mut VM| {
             let len = args[1].as_int()?;
             (0..len)