Added better initial value

Performance is comparable to mp_sqrt now and follows the expected behavior of
the Newton-Raphson algorithm related to time complexity over the full domain.

The initial value has been changed from the hardcoded integer 2 (two) to

    n-th root of a < 2^( floor( ceil(log_2(a)) / n ) + 1)

Further optimizations are possible for the initial value but using more derivatives
for larger numbers (e.g.: Householder) is more effective.