refactor, substitute magic number with readble const variable.
tasking printToInch function; write the test and implement;
change unit's value type from double to Int.
pass new test for printToInch.
implement Length and pass related test.
add new test for Length class.
tasking printReadableLength
add new test for printReadableLength
implement inch's printReadableLength to pass the test.
pass new test for inch's printReadableUnit
implement feet's printReadableUnit and add test.
implement yard's printReadableUnit and  pass the test.
implement mile's printReadableUnit and pass the test.