✨ (slice): Introduce SliceExt trait with has_dup method

[czy-29]

This PR implements a new SliceExt trait with a has_dup method as requested in issue #28.

Changes

• New slice module: Added src/slice.rs with the SliceExt trait
• has_dup method: Efficiently checks for duplicate elements in O(n) average time complexity
• Feature flag: Added slice feature to Cargo.toml following project conventions
• Comprehensive tests: 14 unit tests covering various scenarios including edge cases
• Documentation: Detailed rustdoc with examples and complexity analysis

Implementation Details

The has_dup method uses a HashSet to track seen elements, providing:

• Time Complexity: O(n) average case, O(n²) worst case (rare with good hash function)
• Space Complexity: O(n) for storing seen elements
• Generic Support: Works with any type implementing Hash + Eq

Test Coverage

The implementation includes comprehensive tests for:

• ✅ Slices with duplicates
• ✅ Slices without duplicates
• ✅ Empty slices
• ✅ Single element slices
• ✅ All same elements
• ✅ Different data types (integers, strings, chars)
• ✅ Vec and slice references
• ✅ Large slices (performance verification)
• ✅ Edge cases (duplicates at beginning/end)

Usage Example

use est::slice::SliceExt;

let slice_with_dups = [1, 2, 3, 2, 4];
assert!(slice_with_dups.has_dup());

let slice_without_dups = [1, 2, 3, 4, 5];
assert!(!slice_without_dups.has_dup());

Fixes #28

@czy-29 can click here to continue refining the PR

[czy-29]

@OpenHands 两个问题：

1. 把pub mod slice的文档注释改成：

/// Extensions to the [`std::slice`] module.

2. 两个语言的README里补充上：slice::SliceExt::has_dup()

[openhands-ai]

I'm on it! czy-29 can track my progress at all-hands.dev