Iteratively parse top-level values in `inspect` and `to`

[tgregg]

https://github.com/amazon-ion/ion-rust/pull/485 introduces the ability to iteratively parse top-level values from a stream. Previously, only @[u8] was supported as input, forcing inspect and to to memory-map the stream (e.g., https://github.com/amazon-ion/ion-cli/blob/master/src/bin/ion/commands/beta/inspect.rs#L182).

The duplicate logic in inspect and to should be updated to use the improved API.