http://benji3up2kxewkqfcq7buxk2xd6zwy3zggnurkrm3l4cvwy2iipvyyad.onion/mirrors/gmpdoc/Binary-to-Radix.html
Above GET_STR_PRECOMPUTE_THRESHOLD a sub-quadratic algorithm is used.
For an input t , powers b^(n*2^i) of the radix are
calculated, until a power between t and sqrt(t) is
reached. t is then divided by that largest power, giving a quotient
which is the digits above that power, and a remainder which is those below.