perf: cache parsing for arithmetic expressions

[reubeno]

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[github-actions]

Test Results

    2 files      9 suites   1m 19s ⏱️
  564 tests   564 ✅ 0 💤 0 ❌
1 114 runs  1 114 ✅ 0 💤 0 ❌

Results for commit 84b5ab1.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.52 μs	3.52 μs	0.00 μs	⚪ Unchanged
for_loop	947310.68 μs	582150.94 μs	-365159.74 μs	🟢 -38.55%
instantiate_shell	62.18 μs	62.41 μs	0.24 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	28573.75 μs	30083.47 μs	1509.72 μs	🟠 +5.28%
parse_bash_completion	2801.58 μs	2826.53 μs	24.95 μs	🟠 +0.89%
parse_sample_script	4.22 μs	4.23 μs	0.02 μs	⚪ Unchanged
run_echo_builtin_command	92.43 μs	92.37 μs	-0.06 μs	⚪ Unchanged
run_one_builtin_command	109.76 μs	113.90 μs	4.13 μs	🟠 +3.77%
run_one_external_command	1769.55 μs	1845.45 μs	75.89 μs	🟠 +4.29%
run_one_external_command_directly	838.96 μs	835.74 μs	-3.22 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/openfiles.rs	🟠 51.23%	🔴 47.53%	🔴 -3.7%
brush-core/src/shell.rs	🟢 77.73%	🟢 77.86%	🟢 0.13%
Overall Coverage	🟢 74.81%	🟢 74.78%	🔴 -0.03%

Minimum allowed coverage is 70%, this run produced 74.78%