Does not support JSON serialization using System.Text.Json #26

Description

H3Index instances are unable to be serialized to JSON using System.Text.Json, with JsonSerializer.Serialize(someIndex) resulting in an exception:

System.Text.Json.JsonException : A possible object cycle was detected. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 64. Consider using ReferenceHandler.Preserve on JsonSerializerOptions to support cycles.

Steps to Reproduce

1. Create a H3Index
2. Attempt to serialize it using JsonSerializer.Serialize()
3. Observe stack trace

Expected Result

We should be serializing to/from the hex string representation of the index.