Change 'unsafeDowncast' to 'as!' (#186)

Motivation:

The 'unsafeDowncast' can cause a miscompile leading to unexpected
runtime behaviour.

Modifications:

- Use 'as!' instead

Result:

No miscompiles on 5.10

Sources/ConcurrencyHelpers/Lock.swift

@@ -136,7 +136,8 @@ final class LockStorage<Value>: ManagedBuffer<Value, LockPrimitive> {
         let buffer = Self.create(minimumCapacity: 1) { _ in
             return value
         }
-        let storage = unsafeDowncast(buffer, to: Self.self)
+        // Avoid 'unsafeDowncast' as there is a miscompilation on 5.10.
+        let storage = buffer as! Self
 
         storage.withUnsafeMutablePointers { _, lockPtr in
             LockOperations.create(lockPtr)