(PUP-2382) Convert ADSI GetComputerName to FFI

  - Add missing error handling for GetComputerName Win32 function
  - Add test that verifies GetComputerName returns a non-empty string
(PUP-840) FFI Win32 API calls in colors.rb

 - Replace WriteConsole / WriteConsoleOutputCharacter calls with FFI
   equivalents
(PUP-2382) No need to require 'Win32API' any more