Add parallel batch evaluation API

[Copilot]

Casbin's BatchEnforce API needs efficient parallel evaluation of the same expression against many parameter sets. The existing Eval method is thread-safe but lacks a convenient batch API.

Changes

• New batch evaluation methods

  • EvaluateBatch() - sequential evaluation across parameter sets
  • EvaluateBatchParallel() - parallel evaluation with configurable worker count
  • EvaluationResult struct encapsulating result and error per parameter set

• Comprehensive test coverage

  • Concurrent safety validation (race detector)
  • Batch evaluation with varying worker configurations
  • Error handling in batch operations
  • Casbin-style access control example

• Performance benchmarks

  • Sequential vs parallel batch evaluation
  • Worker count tuning (1, 4, 10, unlimited)

Usage

expression, _ := govaluate.NewEvaluableExpression("(requests_made * requests_succeeded / 100) >= 90")

parameterSets := []map[string]interface{}{
    {"requests_made": 100, "requests_succeeded": 95},
    {"requests_made": 100, "requests_succeeded": 85},
}

// Parallel evaluation with 4 workers
results := expression.EvaluateBatchParallel(parameterSets, 4)

for i, result := range results {
    if result.Error == nil {
        fmt.Printf("Result %d: %v\n", i, result.Result)
    }
}

The parallel version provides better throughput on multi-core systems for large batch sizes. Pass maxWorkers=0 for fully parallel execution or specify a limit to control resource usage.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[feature] support parallel evaluation with good performance</issue_title>
<issue_description>This requirement originates from Casbin's BatchEnforce API: apache/casbin#710

But it can benefit all other projects which want to use this feature</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [feature] support parallel evaluation with good performance #20

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