Use new mapping features of `DataBin`

[ihincks]

What is the expected feature or enhancement?

Once #1690 has been closed, we will have access to the new mapping features of DataBin. We should use those instead of the pseudo private class attributes _FIELDS etc.

Acceptance criteria

• no more reliance on _FIELDS or _FIELD_TYPES in DataBin
• no more access to attributes via getattr

For an instancedata_bin, list(dat_bin) will be equal to _FIELDS, and attribute values can be fetched with getitem syntax.