Change 'unsafeDowncast' to 'as!' (#52)

Motivation:

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

Modifications:

- Use 'as!' instead

Result:

No miscompiles on 5.10

Sources/OpenAPIURLSession/BufferedStream/Lock.swift

@@ -111,7 +111,8 @@ 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)