Support for sequences

Sequences should be allowed in all contexts. Like options and bools, their context will modify how they are parsed. They will be treated as repeated optional values in all cases. If standalone (either by itself or as the value in a newtype variant), it will be parsed as a repeated list of positional optionals. If within a struct, it will be a repeated list of optional values, using the field name as the key.

The standalone version will require some additional parsing logic. The struct version *might* work out of the box by adding it to `optionals`, but some testing should be done to verify that. In both cases, the deserializer itself will need to be modified to support it, both in tracing and in the full deserialization step.