[integer][decimal] Use binary splitting for factorial() + add permutation()

[forfudan]

This PR improves performance of exact factorial computation via balanced binary splitting and introduces a new permutation() API for both BigInt and BigDecimal, with accompanying tests.

Changes:

• Replace naive factorial running-product with a balanced product_range() implementation (binary splitting) for BigInt and exact BigDecimal factorial.
• Add permutation() special functions and public methods on BigInt/BigDecimal (with optional rounding for BigDecimal).
• Add unit tests covering basic permutation() behavior and error handling.

[Copilot]

Pull request overview

This PR improves performance of exact factorial computation via balanced binary splitting and introduces a new permutation() API for both BigInt and BigDecimal, with accompanying tests.

Changes:

• Replace naive factorial running-product with a balanced product_range() implementation (binary splitting) for BigInt and exact BigDecimal factorial.
• Add permutation() special functions and public methods on BigInt/BigDecimal (with optional rounding for BigDecimal).
• Add unit tests covering basic permutation() behavior and error handling.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/bigint/test_bigint_special.mojo	Adds BigInt permutation tests, including k > n and negative k behavior.
tests/bigdecimal/test_bigdecimal_special.mojo	Adds BigDecimal permutation tests for exact and rounded modes.
src/decimo/bigint/special.mojo	Implements factorial via binary splitting and adds permutation() using product_range().
src/decimo/bigint/bigint.mojo	Exposes BigInt.permutation(k) public API delegating to bigint special functions.
src/decimo/bigdecimal/special.mojo	Implements exact factorial via binary splitting and adds exact/rounded permutation().
src/decimo/bigdecimal/bigdecimal.mojo	Exposes BigDecimal.permutation(k, precision) public API delegating to bigdecimal special functions.

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