First version of ecp_mul_comb()
Improve comb method (less precomputed points)
Fix bound issues
Add coordinate randomization back
Optimize w in the comb method
Rm multiplication using NAF

Comb method is at most 1% slower for random points,
and is way faster for fixed point (repeated).
Tighten ecp_mul() validity checks
Small code cleanups
Spare some memory
Add mpi_shrink()
Use mpi_shrink() in ecp_precompute()
Update bibliographic references
Add mpi_safe_cond_assign()
Do point inversion without leaking information
Rm subtraction from ecp_add_mixed()
Misc details
Change method for making M odd in ecp_mul()

- faster
- avoids M >= N (if m = N-1 or N-2)
Make memory access pattern constant
Enhance ecp_selftest
Fix bug in mpi_safe_cond_assign()