change ConcurrentSlab to use Vec

[xunilrj]

Description

Partially addresses #5781.

This reduces the time spent retrieving items in slabs.
For more details see: #5782

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[github-actions]

Benchmark for 79138b3

Click to view benchmark

Test	Base	PR	%
code_action	5.7±0.39ms	5.4±0.16ms	-5.26%
code_lens	294.6±12.85ns	294.9±9.69ns	+0.10%
compile	6.2±0.10s	5.6±0.12s	-9.68%
completion	5.0±0.05ms	4.9±0.05ms	-2.00%
did_change_with_caching	5.9±0.06s	5.4±0.06s	-8.47%
document_symbol	993.0±21.35µs	945.1±9.51µs	-4.82%
format	71.7±0.71ms	71.3±0.66ms	-0.56%
goto_definition	366.3±7.32µs	367.3±6.93µs	+0.27%
highlight	9.4±0.26ms	9.0±0.11ms	-4.26%
hover	684.6±30.85µs	594.1±13.58µs	-13.22%
idents_at_position	123.1±2.36µs	123.8±0.85µs	+0.57%
inlay_hints	670.6±14.11µs	674.2±37.14µs	+0.54%
on_enter	501.7±16.54ns	495.5±26.52ns	-1.24%
parent_decl_at_position	3.6±0.05ms	3.7±0.05ms	+2.78%
prepare_rename	364.8±5.89µs	372.4±8.71µs	+2.08%
rename	9.7±0.13ms	9.5±0.22ms	-2.06%
semantic_tokens	1067.4±16.34µs	1077.8±19.44µs	+0.97%
token_at_position	361.5±5.43µs	357.6±2.79µs	-1.08%
tokens_at_position	3.6±0.06ms	3.7±0.03ms	+2.78%
tokens_for_file	423.3±2.34µs	417.5±3.14µs	-1.37%
traverse	47.0±1.55ms	45.2±1.07ms	-3.83%

[JoshuaBatty]

nice 10% compiler speed up!

[github-actions]

Benchmark for 2182803

Click to view benchmark

Test	Base	PR	%
code_action	5.3±0.03ms	5.4±0.04ms	+1.89%
code_lens	288.2±7.42ns	296.5±10.16ns	+2.88%
compile	6.0±0.11s	5.2±0.09s	-13.33%
completion	4.8±0.03ms	4.9±0.01ms	+2.08%
did_change_with_caching	5.4±0.05s	5.0±0.04s	-7.41%
document_symbol	995.9±49.31µs	975.7±39.84µs	-2.03%
format	71.0±0.76ms	72.1±1.46ms	+1.55%
goto_definition	368.5±5.62µs	357.3±5.66µs	-3.04%
highlight	8.8±0.19ms	9.0±0.16ms	+2.27%
hover	593.4±7.95µs	562.3±8.36µs	-5.24%
idents_at_position	123.4±0.30µs	123.6±0.95µs	+0.16%
inlay_hints	668.2±60.48µs	661.4±7.15µs	-1.02%
on_enter	489.2±13.47ns	504.1±16.38ns	+3.05%
parent_decl_at_position	3.6±0.05ms	3.7±0.05ms	+2.78%
prepare_rename	367.2±5.93µs	357.4±3.74µs	-2.67%
rename	9.2±0.05ms	9.5±0.18ms	+3.26%
semantic_tokens	1049.4±20.43µs	1033.4±14.43µs	-1.52%
token_at_position	350.9±1.04µs	358.4±3.77µs	+2.14%
tokens_at_position	3.6±0.05ms	3.7±0.02ms	+2.78%
tokens_for_file	424.8±2.60µs	416.5±2.50µs	-1.95%
traverse	44.2±1.96ms	43.9±1.30ms	-0.68%