Using typed identifiers instead of strings with the Fluent identifier
enables the diagnostic derive to benefit from the compile-time
validation that comes with typed identifiers - use of a non-existent
Fluent identifier will not compile.
Signed-off-by: David Wood <david.wood@huawei.com>
Remove some duplicated code between both diagnostic derives by
introducing helper functions for reporting an error in case of a invalid
attribute.
Signed-off-by: David Wood <david.wood@huawei.com>
Split `SessionDiagnostic` and `SessionSubdiagnostic` derives and the
various helper functions into multiple modules.
Signed-off-by: David Wood <david.wood@huawei.com>
