stack: Reduce memory allocation of Call
stack: Reduce memory allocation of Call

It does make the code a bit slower but the performance will be improved in a
follow up.

This will also make v2 a bit simpler.

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

Before:
BenchmarkAggregate                  3421            315642 ns/op          116603 B/op       1030 allocs/op
BenchmarkParseDump_Guess             332           3675901 ns/op          664668 B/op       8759 allocs/op
BenchmarkParseDump_NoGuess           345           3463134 ns/op          624355 B/op       8044 allocs/op

After:
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

maruel/panicparse,maruel/panicparse