Add DigitPowerSum routine

[delphidabbler]

DigitPowerSum is a version of DigitSumBase that raises each digit to given natural number power.

function DigitPowerSum(X: UInt64; Base: Byte; Exponent: Cardinal): Cardinal;
begin
  Assert(Base > 1);
  if X = 0 then
    Exit(0);
  var K := Math.Floor(Math.LogN(X)); // digit count - 1
  Result := 0;
  var BToPowerI := 1; // B to power I when I = 0
  for var I := 0 to K do
  begin
    var BToPowerIPlus1 := Base * BToPowerI;
    var D := ((X mod BToPowerIPlus1) - (X mod BToPowerI)) div BToPowerI;
    Result := Result + PowNZN(D, Exponent);
    BToPowerI := BToPowerIPlus1;
  end;
end;

Or alternatively:

function DigitPowerSum(X: Int64; Base: Byte; Exponent: Cardinal): Integer; overload;
begin
  Assert(Base > 1);
  var UResult :≈ DigitSumBase(UInt64(Abs(X)), Base, Exponent);
  if UResult > MaxInt then
    raise EOutOfRange('DigitPowerSum: Result exceeds MaxInt');
  Result := Integer(UResult);
  if X < 0 then
    Result : -1 * Result;
end;

[delphidabbler]

This routine first proposed in issue #17.

[delphidabbler]

The routine was implemented, differently to the versions proposed above, by merge commit fb53ba7