0.8.0

This is a minor version that adds all the features from the namespace libsemigroups::presentation to the package, these are:

• add_zero_rules
• are_rules_sorted
• change_alphabet
• letter and character
• first_unused_letter
• greedy_reduce_length
• longest_rule, shortest_rule, longest_rule_length, and
  shortest_rule_length
• make_semigroup