- prime_count - needs the pc(#) option as well as pc(#,#)

- Do a GMP version of LMO prime_count.  Possible versions:
    - 32-bit main, 16-bit support
    - 64-bit main, 32-bit support   (using __uint64_t if necessary)
    - 128-bit main, 64-bit support  (gcc only)
    - GMP main, 32-bit support      (portable)
    - GMP main, 64-bit support      (mostly portable)

- nth_prime

- GMP SQUFOF could use a better implementation, though low priority since it
  just isn't going to be the right algorithm for numbers > 2^64.  Mainly what
  it needs is to pay attention to the rounds argument.  Perhaps race.

- Add Riemann R function

- Tune and improve SIMPQS for our uses.  Check FLINT 2.3 for improvements.

- Write our own QS.

- The statics in ecm and QS won't play well with threading.

- ECPP: Perhaps more HCPs/WCPs could be loaded if needed?

- ECPP: Another idea is related to Atkin/Morain's EAS.  When we have a large
  number, we can process more Ds, delaying the downrun.  We then use the
  smallest q we found.  Combine with lightened stage 1 factoring as above.
  This drops our q sizes faster, at the expense of more up-front time.
  I have this running, but for small numbers it doesn't matter much, and for
  large numbers it just highlights how much nicer FAS would be.

- ECPP: All discriminants with D % 8 != 3 have been converted to Weber.  We're
  still left with lots of those D values.  Figure out a different invariant
  that will make smaller polynomials, along with a root conversion.

- ECPP: Add a fast BLS5 to downrun?

- Add BLS17 proof.  Merge into BLS5 code since the end is the same.

- Add tests for proofs, similar to MPU t/23.

- Handle objects of type:
     Math::GMP
     Math::GMP::Fast
     Math::GMPz
  We should parse their mpz_t directly, do our processing, and output the
  result as one of these types.

- Recognize Math::BigInt / Math::Pari objects.  Shortcut validation.
  Create results as new objects of their type.

- Pull factor subroutine out of .xs file.  Right now it's impossible to
  implement the various routines that want the factors of n, such as
  divisor_sum, euler_phi, etc.

- These functions should be added:
     divisor_sum
     carmichael_lambda
     liouville
     znprimroot
     euler_phi
     moebius
     jordan_totient
     legendre_phi
     znorder
     znlog

- measure effect of ecpp's Fermat pretest in main as well as ECPP.

- Add is_power to factor, similar to MPU's factor.  Test, from Pari #1074:
   my $n = next_prime(10**20)**200; say join(" ", map {"[@$_]"} factor_exp($n));