PcgRandom Swift 6.1 update

[paiv (🇺🇦 Pavel Ivashkov)]

Quick notes on the package update to Swift 6.1.

• GitHub - paiv/swift-pcg-random: PCG Random for Swift language

Code changes

• Mark Pcg64Random as Sendable
• Use Synchronization Mutex, replacing legacy locking
• Use Swift UInt128, replacing legacy C code
• Move to Swift Testing from XCTest

Compatibility

• Swift 6.1
• macOS 15, iOS 18

In the end updated to less code, thanks new built-in primitives, but for the price of all but latest target platforms.