[integer][decimal] Implement factorial() for BigInt and BigDecimal

[forfudan]

This PR adds factorial support for Decimo’s arbitrary-precision numeric types, exposing factorial() as a standalone function in new special modules and as instance methods on BigInt and BigDecimal.

Changes:

• Added decimo.bigint.special.factorial() and wired it into BigInt.factorial().
• Added decimo.bigdecimal.special.factorial(x, precision=0) (exact and rounded modes) and wired it into BigDecimal.factorial(...).
• Updated module documentation to include the new special modules (and number_theory in the BigInt package doc).

[Copilot]

Pull request overview

This PR adds factorial support for Decimo’s arbitrary-precision numeric types, exposing factorial() as a standalone function in new special modules and as instance methods on BigInt and BigDecimal.

Changes:

• Added decimo.bigint.special.factorial() and wired it into BigInt.factorial().
• Added decimo.bigdecimal.special.factorial(x, precision=0) (exact and rounded modes) and wired it into BigDecimal.factorial(...).
• Updated module documentation to include the new special modules (and number_theory in the BigInt package doc).

Reviewed changes

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

Show a summary per file

File	Description
src/decimo/bigint/special.mojo	Introduces factorial(BigInt) implementation and input cap.
src/decimo/bigint/bigint.mojo	Adds BigInt.factorial() method and imports the new special module.
src/decimo/bigint/init.mojo	Updates BigInt package module listing/documentation.
src/decimo/bigdecimal/special.mojo	Introduces factorial(BigDecimal, precision) with exact and rounded paths.
src/decimo/bigdecimal/bigdecimal.mojo	Adds BigDecimal.factorial(...) method and imports the new special module; minor docstring tweak.

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