math/big: specialize Karatsuba implementation for squaring

Currently we use three different algorithms for squaring:
1. basic multiplication for small numbers
2. basic squaring for medium numbers
3. Karatsuba multiplication for large numbers

Change 3. to a version of Karatsuba multiplication specialized
for x == y.

Increasing the performance of 3. lets us lower the threshold
between 2. and 3.

Adapt TestCalibrate to the change that 3. isn't independent
of the threshold between 1. and 2. any more.

Fixes #23221.

benchstat old.txt new.txt
name           old time/op  new time/op  delta
NatSqr/1-4     29.6ns ± 7%  29.5ns ± 5%     ~     (p=0.103 n=50+50)
NatSqr/2-4     51.9ns ± 1%  51.9ns ± 1%     ~     (p=0.693 n=42+49)
NatSqr/3-4     64.3ns ± 1%  64.1ns ± 0%   -0.26%  (p=0.000 n=46+43)
NatSqr/5-4     93.5ns ± 2%  93.1ns ± 1%   -0.39%  (p=0.000 n=48+49)
NatSqr/8-4      131ns ± 1%   131ns ± 1%     ~     (p=0.870 n=46+49)
NatSqr/10-4     175ns ± 1%   175ns ± 1%   +0.38%  (p=0.000 n=49+47)
NatSqr/20-4     426ns ± 1%   429ns ± 1%   +0.84%  (p=0.000 n=46+48)
NatSqr/30-4     702ns ± 2%   699ns ± 1%   -0.38%  (p=0.011 n=46+44)
NatSqr/50-4    1.44µs ± 2%  1.43µs ± 1%   -0.54%  (p=0.010 n=48+48)
NatSqr/80-4    2.85µs ± 1%  2.87µs ± 1%   +0.68%  (p=0.000 n=47+47)
NatSqr/100-4   4.06µs ± 1%  4.07µs ± 1%   +0.29%  (p=0.000 n=46+45)
NatSqr/200-4   13.4µs ± 1%  13.5µs ± 1%   +0.73%  (p=0.000 n=48+48)
NatSqr/300-4   28.5µs ± 1%  28.2µs ± 1%   -1.22%  (p=0.000 n=46+48)
NatSqr/500-4   81.9µs ± 1%  67.0µs ± 1%  -18.25%  (p=0.000 n=48+48)
NatSqr/800-4    161µs ± 1%   140µs ± 1%  -13.29%  (p=0.000 n=47+48)
NatSqr/1000-4   245µs ± 1%   207µs ± 1%  -15.17%  (p=0.000 n=49+49)

go test -v -calibrate --run TestCalibrate
...
Calibrating threshold between basicSqr(x) and karatsubaSqr(x)
Looking for a timing difference for x between 200 - 500 words by 10 step
words = 200 deltaT =     -980ns (  -7%) is karatsubaSqr(x) better: false
words = 210 deltaT =     -773ns (  -5%) is karatsubaSqr(x) better: false
words = 220 deltaT =     -695ns (  -4%) is karatsubaSqr(x) better: false
words = 230 deltaT =     -570ns (  -3%) is karatsubaSqr(x) better: false
words = 240 deltaT =     -458ns (  -2%) is karatsubaSqr(x) better: false
words = 250 deltaT =      -63ns (   0%) is karatsubaSqr(x) better: false
words = 260 deltaT =      118ns (   0%) is karatsubaSqr(x) better: true  threshold  found
words = 270 deltaT =      377ns (   1%) is karatsubaSqr(x) better: true
words = 280 deltaT =      765ns (   3%) is karatsubaSqr(x) better: true
words = 290 deltaT =      673ns (   2%) is karatsubaSqr(x) better: true
words = 300 deltaT =      502ns (   1%) is karatsubaSqr(x) better: true
words = 310 deltaT =      629ns (   2%) is karatsubaSqr(x) better: true
words = 320 deltaT =    1.011µs (   3%) is karatsubaSqr(x) better: true
words = 330 deltaT =     1.36µs (   4%) is karatsubaSqr(x) better: true
words = 340 deltaT =    3.001µs (   8%) is karatsubaSqr(x) better: true
words = 350 deltaT =    3.178µs (   8%) is karatsubaSqr(x) better: true
...

Change-Id: I6f13c23d94d042539ac28e77fd2618cdc37a429e
Reviewed-on: https://go-review.googlesource.com/105075


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>