nfloat_hypot

With |x| \ge |y| and d the difference of the exponents, the result is |x|(1 + \delta) with 0 < \delta < 2^{-p-3} (p the precision) when d exceeds about half the precision. Otherwise x^2 + y^2 = S \cdot 2^{2e} for an integer S of at most 3n + 2 words (the squares of the mantissas, aligned exactly), whose truncated square root with an inexactness flag gives the result. At one word on 64-bit machines, S is formed in registers and the truncated root is computed from the integer square root of its top two words, since the discarded low bits of S do not change the floor of the square root. At two words, S is formed in registers (the smaller square truncated to the four words of the larger) and its root is approximated with a guard word, which certifies the truncation in all but rare cases. From 12 words (or from 5 words when the exponents differ by at least 3/8 of the precision), the exact squares are replaced by high squarings (flint_mpn_sqrhigh), whose sum is a lower bound within about 4n units of its last word, and the square root with a guard word certifies the truncation likewise, with the exact computation as the fallback.