Optimize immutable vector iterator

[Yu-zh]

Summary

• avoid composing vector iteration through Tree::iter, FixedArray::iter, and Iter::concat
• implement Vector::iter as a single state machine over the tree prefix and tail
• add focused vector iteration benchmarks

Benchmarks

Native backend, n=100000:

case	before	after	speedup
iter().fold	1.34 ms	723.81 us	1.85x
iter().each	1.33 ms	717.15 us	1.85x
iter().to_array	1.36 ms	785.01 us	1.73x
for x in vector	1.33 ms	726.79 us	1.83x

Validation

• moon test immut/vector --target all --target-dir /tmp/core-vector-iter-test
• moon check immut/vector --target all --target-dir /tmp/core-vector-iter-check
• moon bench --package immut/vector --file vector_iter_bench_test.mbt --target native --target-dir /tmp/core-vector-iter-bench-before
• moon bench --package immut/vector --file vector_iter_bench_test.mbt --target native --target-dir /tmp/core-vector-iter-bench-after2
• moon fmt
• moon info
• git diff --check

[coveralls]

Coverage Report for CI Build 4288

Coverage increased (+0.001%) to 94.648%

Details

• Coverage increased (+0.001%) from the base build.
• Patch coverage: 18 of 18 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	15733
Covered Lines:	14891
Line Coverage:	94.65%
Coverage Strength:	218117.47 hits per line

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💛 - Coveralls