Add a function for computing multiplicative order

[davidamarquis]

Benchmark

@btime bench_mult_order_es1(1009)
210.208 μs (8056 allocations: 188.81 KiB)

@btime bench_mult_order(1009)
727.166 μs (28 allocations: 1.05 KiB

function mult_order_eric1(n::Int, q::Int)
    (q <= 0 || n <= 0) && 
        throw(DomainError("q and n both need to be positive. Passed: q = $q, n = $n"))

    # finite stop instead of while
    for i in 1:200
        if mod(BigInt(q)^i, n) == 1
            return i
        end
    end
    error("Unable to compute ord($n, $q).")
end

function bench_mult_order_eric1(n::Int)
    for x in 2:(n-1) 
        mult_order_eric1(x,n) 
    end 
end

function bench_mult_order(n::Int)
    data = prepare_mult_order(n)
    for x in 2:(n-1)
        mult_order(x, data) 
    end 
end