Try to call Less() fewer times when iterating

[a180285]

Base on issue #42

Cached stop index when iterating. And following is some benchmark compare result on my PC.

Benchmarck

  goos: windows
  pkg: github.com/google/btree
  cpu: Intel(R) Core(TM) i5-6600 CPU @ 3.30GHz

- without cache
+ after cache stopIndex

  goarch: amd64 go1.18
- BenchmarkAscendRangeG-4            14937             74071 ns/op
+ BenchmarkAscendRangeG-4            22119             55385 ns/op

- BenchmarkDescendRangeG-4           12909             92070 ns/op
+ BenchmarkDescendRangeG-4           15200             78136 ns/op

- BenchmarkAscendRange-4              9895            122200 ns/op
+ BenchmarkAscendRange-4             14628             81924 ns/op

  goarch: amd64 go1.17
- BenchmarkAscendRange-4             12496             94311 ns/op
+ BenchmarkAscendRange-4             17521             68965 ns/op

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[a180285]

CLA check has passed: https://github.com/google/btree/pull/49/checks?check_run_id=10372524044

[a180285]

Hi @gconnell , could you help reviewing this merge request?

[a180285]

@gconnell @gauntface can you help review this? thank you.