GitLab

- localize some functionality, std::array for digest internals.
  Additional append sub-string methods, pass-through write of digest
  etc.

- the resulting SHA1 can be written and parsed directly
  without any ambiguities when it starts with a digit
  eg, SHA1: _024ea2fa570968069f52f55aa47ee82ac417f5f6

- provide same optional prefix functionality in conversion to string
  and in comparison with strings

- add SHA1Digest::null

- eases management/control of previously calculated sums
- add empty() member function to SHA1Digest
- adjust Copyright dates

- can use 'XX.empty()' instead of 'XX.size() == 0', 'XX.size() < 1' or
  'XX.size() <= 0' or for simpler coding.
  It also has the same number of characters as '!XX.size()' and /might/ be
  more readable

- many size checking had 'XX.size() > 0', 'XX.size() != 0', or 'XX.size() >= 1'
  when a simple 'XX.size()' suffices