Re: Optimize numeric.c mul_var() using the Karatsuba algorithm

The following review has been posted through the commitfest application:
make installcheck-world: not tested
Implements feature: not tested
Spec compliant: not tested
Documentation: not tested

This applies cleanly to master, builds and passes regression tests in my Windows/Cygwin environment.

I also read through comments and confirmed for myself that the assumption about the caller ensuring var1 is shorter is done already in unchanged code from mul_var. Frees correspond to inits. The "Karatsuba condition" reasoning for deciding whether a number is big enough to use this algorithm appears to match what Joel has stated in this thread.

The arithmetic appears to match what's been described in the comments. I have *not* confirmed that with any detailed review of the Karatsuba algorithm from outside sources, other implementations like the Rust one referenced here, or anything similar. I'm hoping that the regression tests give sufficient coverage that if the arithmetic was incorrect there would be obvious failures. If additional coverage was needed, cases falling immediately on either side of the limiting conditions used in the patch would probably be useful. From the limited precedent I've exposed myself to, that doesn't seem to be required here, but I'm open to contrary input from other reviewers. In the meantime, I'm marking this approved.

Thanks for the detailed background and comments, Joel!

The new status of this patch is: Ready for Committer