UJOTYP-47: add timestamp variant type to serialization
- handle type 'UJO_TYPE_TIMESTAMP' in function 'ujot_read_variant_from_ujo'
- handle type 'UJOT_VARIANT_TYPE_TIMESTAMP' in function 'ujot_write_variant_to_ujo'
- make read function aware of UJO null types
- change value of constant 'UJOT_VARIANT_TYPE_TIMESTAMP' to be in sync with UJO
- refactor function 'ujot_on_parsed_ujo_element' to get it smaller
- change 'ujot_variant_new_timestamp' to handle null variant type
- ensure that the the padding bytes in timestamp structure are zero
- change 'ujot_variant_as_timestamp' for returning a borrowed reference to the timestamp data
- ...instead of copying the timestamp data to a given structure
- adapt the module 'test_variant_timestamp.c' to the 'ujot_variant_as_timestamp' change
- add variant type 'UJOT_VARIANT_TYPE_TIMESTAMP' to test structure generator
- add variant type 'UJOT_VARIANT_TYPE_TIMESTAMP' to test structure verifier
- add timestamp constant to the test data
- enhance the test structure by timestamp variants
- add test functions 'ujot_test_for_legal_pointer', 'ujot_test_for_null_pointer' and 'ujot_test_equal_timestamp_value'