Log Message:
snd: Update to 20.4

Snd 20.4:

s7: added s7_scheme* argument to s7_is_eqv, and all the s7_set_*_function calls that
      set optimizer functions (the preceding calls were, sadly, not thread-safe).
    libarb_s7.c: a multiprecision library paralleling GSL.  Requires libflint.
      Only in the gmp version of s7.

checked: ubuntu 20.04, sbcl-2.0.4, FC 32, gsl 2.6, Ruby 2.7, gcc/clang 10

Thanks!: Elijah, Woody Douglass, Ian Duncan, Kjetil Matheussen