Currently, the multisig descriptor constructions are limited to 16 keys only possibly because of template parsing reasons. Under p2sh context, such scripts would be invalid anyway because of 520-byte rule, but we can support up to 20 keys for CHECKMULTISIG in witness script context. 


* multiplies that polynomial by x, computes its remainder modulo a generator, and adds the constant term val.
 *
 * This generator is G(x) = x^8 + {30}x^7 + {23}x^6 + {15}x^5 + {14}x^4 + {10}x^3 + {6}x^2 + {12}x + {9}.
 * It is chosen to define an cyclic error detecting code which is selected by:
 * - Starting from all BCH codes over GF(32) of degree 8 and below, which by construction guarantee detecting
 *   3 errors in windows up to 19000 symbols.
 * - Taking all those generators, and for degree 7 ones, extend them to degree 8 by adding all degree-1 factors.
 * - Selecting just the set of generators that guarantee detecting 4 errors in a window of length 512.
 * - Selecting one of those with best worst-case behavior for 5 errors in windows of length up to 512.
 *
 * The generator and the constants to implement it can be verified using this Sage code:
 *   B = GF(2) # Binary field
 *   BP.<b> = B[] # Polynomials over the binary field
 *   F_mod = b**5 + b**3 + 1
 *   F.<f> = GF(32, modulus=F_mod, repr='int') # GF(32) definition
 *   FP.<x> = F[] # Polynomials over GF(32)
 *   E_mod = x**3 + x + F.fetch_int(8)
 *   E.<e> = F.extension(E_mod) # Extension field definition
 *   alpha = e**2743 # Choice of an element in extension field
 *   for p in divisors(E.order() - 1): # Verify alpha has order 32767.
 *       assert((alpha**p == 1) == (p % 32767 == 0))
 *   G = lcm([(alpha**i).minpoly() for i in [1056,1057,1058]] + [x + 1])
 *   print(G) # Print out the generator
 *   for i in [1,2,4,8,16]: # Print out {1,2,4,8,16}*(G mod x^8), packed in hex integers.
 *       v = 0
 *       for coef in reversed((F.fetch_int(i)*(G % x**8)).coefficients(sparse=True)):
 *           v = v*32 + coef.integer_representation()
 *       print("0x%x" % v)
 */
uint64_t PolyMod(uint64_t c, int val)