[SoM] API review

[rolfbjarne]

The public SwiftRuntimeLibrary API needs a review.

Things to review:

• Every API needs a test.
• Check if classes should be sealed.
• Improve parameter names (#1346).
• Try to reduce code duplication as much as possible.
• Simplify unsafe code as much as possible; complete removal being the best outcome.
• Simplify P/Invokes by using Unsafe[Mutable]Pointers, which do not need retains/releases.
• Check API if there's something obvious (from a .NET perspective) missing. Example: I added a Capacity getter to SwiftArray.
• Throw ObjectDisposedException when needed.
• Add parameter validation.

Example review for SwiftArray: bf54aaa528c64be5b1d1e6eb97e1e59f493c8002