Implement constraints on what data a tuple can contain

A tuple can be stored in more compact form if, for example, we know that all fields in it are 4-byte ints.
Provide user with a way to specify it, and use such constraints to speed up storage and update.

Another case of a desirable constraint is when it's necessary to require that all or part of fields in a tuple are unique.
This can be useful if we want to store a set or a unique queue in a tuple.