Added constant generator combinator
Added constant generator combinator

paulyoung/FsCheck,jhamm/FsCheck,vasily-kirichenko/FsCheck,kos59125/FsCheck,forki/FsCheck,paulyoung/FsCheck,fscheck/FsCheck,dungpa/FsCheck,ErikSchierboom/FsCheck,yukitos/FsCheck,yukitos/FsCheck,cloudRoutine/FsCheck,sideeffffect/FsCheck,ploeh/FsCheck,fsharp/FsCheck,yukitos/FsCheck,fscheck/FsCheck,cloudRoutine/FsCheck,forki/FsCheck