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

Avoids the clutter and maintenance effort associated with providing the
function signature string.

- 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

  - add static/protected member function subsections
  - static members up front, constructors/destructors next
    private/protected/public members next