Struct-writer

- Revert back to 0.4.8 functionality

- Fix union size calculation

- Seperate out union definition from the group structure to enable support for languages that do not allow anonymous unions embedded in the structure.

- Fall back to returning bytes when we fail to parse data

Features
- Support python 3.11 - 3.12

Fixes
- Handle enums with negative values
- Support signed integers in bitfields

Change
- Remove the _u from the group union structure name