Numerous Fixes For Chained Operations And Performance Improvements

[mergesort]

Store

When using a chained operation it was possible for not all values to be removed properly, leading to the incorrect storage of extra data.

try await self.$items
    .remove(oldItems)
    .insert(newItems)
    .run()

More tests have been added to test all sorts of chaining scenarios to prevent this regression from occurring again.

SecurelyStoredValue

When you had a keychain value which existed but it's shape changed (such as adding or removing a property from a type), it was impossible to remove that value. Now the .remove() function will remove a value when it cannot properly decode the old value, allowing you to overwrite values when adding/removing properties or changing the underlying type of a SecurelyStoredValue.