Chore: use Self in more places in vectors

[connortsui20]

This makes it easier to read the code since the immutable vs mutable names are so similar.

This is a purely cosmetic change

[codecov]

Codecov Report

❌ Patch coverage is 26.89655% with 106 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.87%. Comparing base (28f5b3d) to head (b8364c6).
⚠️ Report is 8 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-vector/src/primitive/vector_mut.rs	20.00%	44 Missing ⚠️
vortex-vector/src/decimal/vector_mut.rs	13.33%	26 Missing ⚠️
vortex-vector/src/primitive/vector.rs	50.00%	17 Missing ⚠️
vortex-vector/src/decimal/vector.rs	0.00%	13 Missing ⚠️
vortex-vector/src/decimal/generic.rs	0.00%	2 Missing ⚠️
vortex-vector/src/struct_/vector.rs	33.33%	2 Missing ⚠️
vortex-vector/src/bool/vector.rs	50.00%	1 Missing ⚠️
vortex-vector/src/primitive/generic.rs	50.00%	1 Missing ⚠️

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