ezgliding / goigc

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Improve Task.Distance by not allocating new slices

pprof analysis showed the following results:

     0.95s  2.13%  3.01%     30.22s 67.86%
github.com/ezgliding/goigc.(*Task).Distance
     2.54s  5.70%  8.71%     16.31s 36.63%  runtime.growslice
     3.56s  7.99% 16.71%     15.55s 34.92%  runtime.mallocgc
     0.48s  1.08% 17.79%     11.55s 25.94%
github.com/ezgliding/goigc.(*Point).Distance

which shows a big issue with slice allocation between the Task.Distance
and Point.Distance calls. Looking at the code, probably for easy of use,
we were creating a new slice on *every* Distance calculation.

Use the Points already in the task instead. Here's the result:

benchcmp old.txt new.txt
benchmark                                                   old ns/op
new ns/op       delta
BenchmarkBruteForceOptimize/optimize-short-flight-1/1-4     42654178724
21424710602     -49.77%

4 of 4 new or added lines in 1 file covered. (100.0%)

378 of 438 relevant lines covered (86.3%)

1991015.31 hits per line