Allow for renaming struct/union fields

I've added a `ParseCallbacks` method called `field_name` which is given
the same `FieldInfo` struct that `field_visibility` takes.

I wasn't entirely sure where to change field names, I ended up doing it
in what was `deanonymize_fields` which has been renamed to
`assign_field_names` as suggested by [this
comment](#1058 (comment))

Related to #1097 and #1098

Author: wolfd