Fix: incorrect FromIterator<bool> implementation

[connortsui20]

It incorrectly assumed that the upper bound for iterators is true.

Also adds a set_len method.

[codspeed-hq]

CodSpeed Performance Report

Merging #5056 will degrade performances by 18.21%

_{Comparing ct/from-iter-bits (9781fc0) with develop (469b801)}

Summary

❌ 2 regressions
✅ 1307 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	from_iter[BitBuffer, 16384]	34.9 µs	41.8 µs	-16.41%
❌	from_iter[BitBuffer, 65536]	122.8 µs	150.2 µs	-18.21%

[codecov]

Codecov Report

❌ Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.52%. Comparing base (469b801) to head (9781fc0).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-buffer/src/bit/buf_mut.rs	82.22%	8 Missing ⚠️

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[connortsui20]

We get a small regression here (likely due to the extra branch) and it also regresses on my macbook, but AFAIK this is the cost of correctness here (and the benchmark is the idealized version where we know the exact length of the iterator anyways)