fix(arithmetic): recursively evaluate var references

[reubeno]

In an arithmetic expression, we need to recursively parse and evaluate the body of any bare variable references.

Resolves #345.

[github-actions]

Test Results

    2 files      9 suites   1m 29s ⏱️
  623 tests   623 ✅ 0 💤 0 ❌
1 232 runs  1 232 ✅ 0 💤 0 ❌

Results for commit 882769f.

♻️ This comment has been updated with latest results.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	16.05 μs	16.67 μs	0.62 μs	🟠 +3.89%
eval_arithmetic	0.19 μs	0.19 μs	0.00 μs	⚪ Unchanged
expand_one_string	1.51 μs	1.57 μs	0.06 μs	⚪ Unchanged
for_loop	32.06 μs	34.67 μs	2.61 μs	🟠 +8.13%
function_call	3.29 μs	3.31 μs	0.02 μs	⚪ Unchanged
instantiate_shell	48.27 μs	48.75 μs	0.48 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	21565.22 μs	22497.96 μs	932.74 μs	🟠 +4.33%
parse_bash_completion	1667.39 μs	1654.30 μs	-13.10 μs	⚪ Unchanged
parse_sample_script	1.74 μs	1.74 μs	-0.00 μs	⚪ Unchanged
run_echo_builtin_command	15.77 μs	15.77 μs	-0.00 μs	⚪ Unchanged
run_one_external_command	4105.12 μs	2265.83 μs	-1839.29 μs	🟢 -44.80%
tokenize_sample_script	2.88 μs	2.84 μs	-0.03 μs	🟢 -1.18%

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/arithmetic.rs	🟢 98.46%	🟢 98.47%	🟢 0.01%
brush-core/src/builtins/dot.rs	🟢 91.67%	🟢 95.83%	🟢 4.16%
brush-core/src/builtins/return_.rs	🟢 81.82%	🟢 100%	🟢 18.18%
brush-core/src/shell.rs	🟢 77.62%	🟢 78.34%	🟢 0.72%
Overall Coverage	🟢 74.7%	🟢 74.77%	🟢 0.07%

Minimum allowed coverage is 70%, this run produced 74.77%