_flint_mpn_tdiv_qr_small

Euclidean division for 1 \le \mathit{bn} \le FLINT_MPN_DIV_SMALL_BN (7), with the conventions of flint_mpn_tdiv_qr (r may be NULL; no normalisation of b is needed). There is one code path per divisor length, in which the normalised divisor and the partial remainder are kept in registers and the dividend is normalised on the fly; each quotient limb takes a 3/2 division (2/1 for \mathit{bn} = 1) and, for \mathit{bn} \ge 3, the subtraction of a 1 \times (\mathit{bn} - 2) product, as in GMP's mpn_sbpi1_div_qr but without function calls or memory traffic. The cost is linear in \mathit{an}; beyond about 3\mathit{bn} limbs the schoolbook division is faster for \mathit{bn} \ge 4.