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### `udiv`, `urem`, `urembymul`, `urembymul_precomp`, `ugcd`

`udiv(p1,p2)`
`urem(p1,p2)`
`urembymul(p1,p2)`
`urembymul_precomp(p1,p2,inv)`
`ugcd(p1,p2)`
  :: Division and GCD for univariate polynomials.

return
  univariate polynomial
p1 p2 inv
  univariate polynomial

- For univariate polynomials p1 and p2, there exist polynomials q and r such that p1=q*p2+r and the degree of r is less than that of p2. Then `udiv` returns q, `urem` and `urembymul` return r. `ugcd` returns the polynomial GCD of p1 and p2. These functions are specially tuned up for dense univariate polynomials. In `urembymul` the division by p2 is replaced with the inverse computation of p2 as a power series and two polynomial multiplications. It speeds up the computation when the degrees of inputs are large.
- `urembymul_precomp` is efficient when one repeats divisions by a fixed polynomial. One has to compute the third argument by `ureverse_inv_as_power_series()`.

    [177] setmod_ff(2^160-47);
    1461501637330902918203684832716283019655932542929
    [178] A=randpoly_ff(200,x)$
    [179] B=randpoly_ff(101,x)$
    [180] cputime(1)$
    0sec(1.597e-05sec)
    [181] srem(A,B)$
    0.15sec + gc : 0.15sec(0.3035sec)
    [182] urem(A,B)$
    0.11sec + gc : 0.12sec(0.2347sec)
    [183] urembymul(A,B)$
    0.08sec + gc : 0.09sec(0.1651sec)
    [184] R=ureverse_inv_as_power_series(B,101)$
    0.04sec + gc : 0.03sec(0.063sec)
    [185] urembymul_precomp(A,B,R)$
    0.03sec(0.02501sec)

`References`
  section `uinv_as_power_series`, `ureverse_inv_as_power_series`.

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