Replace `ByteString(ByteArray)` constructor with `ByteArray.toByteString()`

[joffrey-bion]

This current "safe" constructor doesn't feel right. It has the following drawbacks:

• the constructor syntax implies "wrapping" (as if the array is used as a property), but what actually happens is a copy
• it's hard to discover: when I want to turn a ByteString into a ByteArray, the completion helps me find the toByteArray() extension. When I look for the inverse operation, I can't find it via completion
• it's not symmetrical with the ByteString.toByteArray() counterpart
• it's not easily composable: nullableByteString?.toByteArray() is nice, while nullableByteArray?.let { ByteString(it) } is not nice

Adding a ByteArray.toByteString() extension would solve all of these.