Sudoku solver in Swift

I wrote this while on holiday in Swift Playground on iPad. That original code is available at:

• Original blog post
• Source w/ Value Types
• Source w/ Reference Types

This is that code but run as a Swift Package, timed with XCTest in RELEASE (with a few small improvements).

• Updated blog post

Timings

|        |   iPAD Playground  |    MAC #RELEASE   |    +Improvements   |
|        |    VAL   |   REF   |   VAL    |  REF   |  VAL   |    REF    |
|--------|----------|---------|----------|--------|--------|-----------|
|   Easy |      5 |     6 |   0.09 |  0.3 |  0.1ms |     0.4ms |
| Medium |    187ms |     9ms |   1.50ms |  0.5ms |  0.3ms |     0.5ms |
|   Hard |     52ms |    12ms |   0.50ms |  0.7ms |  0.2ms |     0.9ms |
| Expert | 31_551ms | 1_486ms | 189.00ms | 63.0ms | 32.0ms | 110-1.1ms |