TODO for 5.2

[Torbjorn Granlund]

This is what I want to fix before the 5.2 release.  Please remind me if
I have forgotten something.

* Strongly consider making mpn_sec_div_qr return high quotient limb and
  write just nn-dn quotient limbs to qp area.

* Finalise and commit mpn_sec_minvert.
* Add some other sec functions from Niels' list.

* Split up outer loops of mpn/x86_64/coreisbr/mul_basecase.asm.
* Split up outer loops of mpn/x86_64/coreihwl/mul_basecase.asm.
* (More similar project at https://gmplib.org/devel/x64-64.html.)

* Make sure the test suite exercises new sec code.

* Run tuneup with large limits for critical machines, update many
  gmp-mparam.h files.


Torbjörn