implement TProxyAggregateStream (Delphi compatibility)

See: https://docwiki.embarcadero.com/Libraries/Athens/en/System.Classes.TProxyAggregateStream

Here is a test for the test suite:

program ProxyAggregateStreamTest;

uses
  Classes, SysUtils;

var
  P: TProxyAggregateStream;
  S: RawByteString;
  I: Integer;
  C: AnsiChar;
begin
  P := TProxyAggregateStream.Create;
  try
    P.AddStream(TRawByteStringStream.Create('0123'), True);
    P.AddStream(TRawByteStringStream.Create('45'), True);
    P.AddStream(TRawByteStringStream.Create('6'), True);
    P.AddStream(TRawByteStringStream.Create('789'), True);

    // test concat read
    P.Position := 0;
    SetLength(S, 10);
    if not((P.Read(S[1], 10)=10) and (S='0123456789')) then
      Halt(1);

    // test seek to position
    for I := 0 to P.Size-1 do
    begin
      P.Position := I;
      if not((P.Read(C, 1)=1) and (C=IntToStr(I))) then
        Halt(2);
    end;

    // test seek from end
    P.Seek(-1, soEnd);
    if not((P.Read(C, 1)=1) and (C=IntToStr(9))) then
      Halt(3);

    // test remove stream
    P.RemoveStream(1);
    P.Position := 3;
    SetLength(S, 4);
    if not((P.Read(S[1], 4)=4) and (S='3678')) then
      Halt(4);
  finally
    P.Free;
  end;
end.