Tail optimization for RRB-Vector (immut/vector) and new operations

[CAIMEOX]

This PR deprecates immut/array and adds a new implementation immut/vector, a tail-backed persistent vector with tree-local range operations and a tail-aware concat path.

• switch the persistent layout to tree + tail
• add peek, pop, split, take, drop, and slice
• optimize concat with a tail-aware concat_with_suffix path

I also ran the benchmark. The delta below shows the change in this pull request relative to the base. A negative number indicates that the new version is faster. All values are in milliseconds.

case	size × iters	new	base	delta
new	0 × 200000	1.310	1.397	-6.2%
make	65536 × 64	0.586	0.672	-12.9%
makei	65536 × 32	3.572	6.830	-47.7%
from_array	65536 × 32	6.818	8.316	-18.0%
from_iter	65536 × 32	5.497	5.676	-3.2%
to_array	65536 × 32	5.253	5.228	+0.5%
length	65536 × 500000	2.132	2.257	-5.5%
is_empty	65536 × 500000	2.058	2.210	-6.9%
at	65536 × 65536	0.741	0.849	-12.8%
get	65536 × 65536	0.695	0.803	-13.5%
set	65536 × 4096	0.565	0.666	-15.2%
push	65536 × 20	31.183	123.366	-74.7%
concat	32768 × 1024	1.968	2.043	-3.7%
iter	65536 × 32	10.654	10.482	+1.6%
each	65536 × 32	8.749	8.783	-0.4%
eachi	65536 × 16	8.539	8.268	+3.3%
fold	65536 × 32	1.632	1.739	-6.1%
rev_fold	65536 × 32	1.621	1.762	-8.0%
map	32768 × 16	0.751	0.812	-7.5%

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[coveralls]

Pull Request Test Coverage Report for Build 3531

Details

• 498 of 610 (81.64%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.6%) to 95.014%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
immut/vector/utils.mbt	29	31	93.55%
immut/vector/tree_pop.mbt	17	21	80.95%
immut/vector/tree_utils.mbt	21	25	84.0%
immut/vector/tree_append.mbt	44	59	74.58%
immut/vector/tree_slice.mbt	16	34	47.06%
immut/vector/vector.mbt	190	208	91.35%
immut/vector/tree.mbt	178	229	77.73%

Totals
Change from base Build 3527:	-0.6%
Covered Lines:	14598
Relevant Lines:	15364

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💛 - Coveralls

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[bobzhang]

@codex do a review

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