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

- Consider adding Lehmer's method for prime count.  The only use I can really
  think of would be 32-bit machines.  I worry that the overhead of GMP would
  kill us, and some method using __uint64s, or even Math::Int64 would be
  faster.

- 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: If we are in step M, saying "for this N, if Q is prime then N is
  prime", and Q comes back from downrun as composite, then we should
    1: Run more compositeness tests (e.g. frobenius_underwood or some m-rs)
    2: Continue on trying to find a different Q.
  This does not automatically mean N is composite (though it would indicate
  we found a BPSW counterexample).  Make sure we're doing the right thing.

- 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.