Add Unit, Union, Option as comparable types

This MR adds unit as a comparable type and closes comparable types under option and or.

Comparing Units always return 0, but it's useful when unit appears in a more complex type.

For options, None is smaller than Somes.

For unions, Lefts are smaller than Rights.

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This is a redo of the third part of nomadic-labs/tezos!131 and is based on cryptiumlabs/tezos!65

Tasks:

• Merge cryptiumlabs/tezos!65
• Rebase
• Tests
• Documentation