stack: increase performance
stack: increase performance

Always allocate a minimum of 4 []*Goroutine, 4 []raceOp, 4 []Call and
4[]Value. It makes a difference in performance. One problem this surfaced is
that the benchmarks are too trivial right now to show the improvement.

go test -bench=. -cpu 1 ./stack

Before:
BenchmarkAggregate                  3351            337869 ns/op          116676 B/op       1034 allocs/op
BenchmarkParseDump_Guess             330           3642024 ns/op          603923 B/op       8337 allocs/op
BenchmarkParseDump_NoGuess           351           3415569 ns/op          563743 B/op       7623 allocs/op

After:
BenchmarkAggregate                  3385            347734 ns/op          116646 B/op       1032 allocs/op
BenchmarkParseDump_Guess             342           3513786 ns/op          581431 B/op       7685 allocs/op
BenchmarkParseDump_NoGuess           352           3403213 ns/op          541022 B/op       6970 allocs/op

maruel/panicparse,maruel/panicparse