Rename module to SwiftBigInt to fix module stability compilation

[jlalvarez18]

Rename module to SwiftBigInt to fix module stability compilation

BREAKING CHANGE: Module name changed from BigInt to SwiftBigInt

Problem

The module name "BigInt" conflicted with the struct name "public struct BigInt",
causing Swift compiler errors when building with BUILD_LIBRARY_FOR_DISTRIBUTION=YES
(required for XCFramework distribution and module stability):

error: 'BigInt' is not a member type of struct 'BigInt.BigInt'

Solution

Renamed the module to "SwiftBigInt" following Swift community conventions
(similar to SwiftNIO, SwiftLog, SwiftCrypto).

Changes

• Package name: Remains "BigInt" (matches repository name)
• Product name: BigInt → SwiftBigInt
• Module name: BigInt → SwiftBigInt
• Type names: UNCHANGED (BigInt and BigUInt structs remain the same)

Migration for Consumers

Only import statements need to change:

// Before (v5.x):
import BigInt

// After (v6.0+):
import SwiftBigInt

// Type names unchanged:
let x = BigInt(100)   // ✓ Still works
let y = BigUInt(50)   // ✓ Still works

Benefits

• ✅ Fixes module stability compilation errors
• ✅ Enables XCFramework distribution without compiler workarounds
• ✅ Eliminates need for -no-verify-emitted-module-interface flag
• ✅ Follows Swift community naming conventions
• ✅ Minimal breaking changes (import statements only)

Files Modified

• Package.swift: Updated module and product names
• All test files (18 files): Updated imports
• Demo playground files (5 files): Updated imports
• README.md: Added migration guide
• Documentation: Updated import examples

Version

Requires major version bump: 5.4.0 → 6.0.0

Fixes module interface verification bug when building with library evolution enabled.

[tgymnich]

Thank you for looking into this, I think it would be better to wait for Swift to implement module selectors swiftlang/swift#34556 which will most likely land in the next Swift release.