Converts specific errors to RpcNetworkError, which can be bubbled to applications at a higher level and retried on the network (eg with exponential backoff) until resolved or timed out.
* chore: export subscriptions
This has a knock-on effect of nim-serde not being imported into subscriptions when JsonRpcProvider.new is called from a consumer that does not export nim-serde.
* import/export serde
* Replace all instances of std/json with pkg/serde
