_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 $\mathit{bn} \ge 4$.