Add go1.23 iterators

[sylr]

$ go test -bench='BenchmarkAscend(|Seq|Seq2)G$' -test.benchmem -count=6 -test.benchtime=20000x -test.memprofile allocs.prof
goos: darwin
goarch: arm64
pkg: sylr.dev/btree/v2
cpu: Apple M2
BenchmarkAscendG-8       	   20000	     32667 ns/op	       0 B/op	       0 allocs/op
BenchmarkAscendG-8       	   20000	     31972 ns/op	       0 B/op	       0 allocs/op
BenchmarkAscendG-8       	   20000	     32045 ns/op	       0 B/op	       0 allocs/op
BenchmarkAscendG-8       	   20000	     31884 ns/op	       0 B/op	       0 allocs/op
BenchmarkAscendG-8       	   20000	     32027 ns/op	       0 B/op	       0 allocs/op
BenchmarkAscendG-8       	   20000	     31839 ns/op	       0 B/op	       0 allocs/op
BenchmarkAscendSeqG-8    	   20000	     42069 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeqG-8    	   20000	     41595 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeqG-8    	   20000	     41571 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeqG-8    	   20000	     41353 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeqG-8    	   20000	     41462 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeqG-8    	   20000	     41467 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeq2G-8   	   20000	     34653 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeq2G-8   	   20000	     34521 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeq2G-8   	   20000	     34524 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeq2G-8   	   20000	     34748 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeq2G-8   	   20000	     34456 ns/op	     104 B/op	       4 allocs/op
BenchmarkAscendSeq2G-8   	   20000	     34478 ns/op	     104 B/op	       4 allocs/op
PASS
ok  	sylr.dev/btree/v2	13.211s

[codecov]

Codecov Report

Attention: Patch coverage is 7.14286% with 65 lines in your changes missing coverage. Please review.

Project coverage is 68.48%. Comparing base (60bdda9) to head (afaf957).

Files with missing lines	Patch %	Lines
btree.go	7.14%	64 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (60bdda9) and HEAD (afaf957). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (60bdda9)	HEAD (afaf957)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master       #5       +/-   ##
===========================================
- Coverage   79.29%   68.48%   -10.81%     
===========================================
  Files           1        1               
  Lines         425      495       +70     
===========================================
+ Hits          337      339        +2     
- Misses         75      141       +66     
- Partials       13       15        +2     

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