refactor(tvix/eval): remove Value::Json and related functionality

Currently Value::Json is used in combination with VMRequest::ToJson to
recursively convert tvix Value to serde_json::Value. This functionality
is used in builtins.toJSON as well as derivation __structuredAttrs.

Both Value::Json and VMRequest::ToJson were removed in this commit.

Related functionality in vm.rs is also removed: vm.rs does not know
about JSON anymore.

Recursively converting to serde_json now happens without going through
the VM.

Thrown errors that are part of the value of toJSON are now directly
propagated as ErrorKind, were-as previously there was a split between
CatchableErrorKind and ErrorKind, where eventually CatchableErrorKind
would be converted to ErrorKind::Catchable.

Change-Id: I066f064926c491e4c087a984f07af43d19124cfe
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12732
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/serde/src/de.rs

@@ -105,7 +105,6 @@ impl<'de> de::Deserializer<'de> for NixDeserializer {
             | Value::Blueprint(_)
             | Value::DeferredUpvalue(_)
             | Value::UnresolvedPath(_)
-            | Value::Json(..)
             | Value::Catchable(_)
             | Value::FinaliseRequest(_) => Err(Error::Unserializable {
                 value_type: self.value.type_of(),