Support Remaining Ion Types

[dlurton]

I discovered today that round-tripping from Ion to partiql_value::Value using partiql_extension_ion and back again converts all symbols to strings. I know that full support for Ion is incomplete, but I don't see an issue for finishing this, so I am adding one now.

Support for any remaining Ion types must also be included in the partiql_extension_ion so round-tripping described above is not lossy.

[dlurton]

Also, to be clear, typed nulls are part of this too.

AFAIK the list for full Ion support is now:

• sexp type
• symbol type
• clob type
• big integers
• typed nulls