Make Unit type less ambiguous

[carte7000]

Given a contract with Unit as storage

Currently originating such a contract with null (or any truthy value) in the storage property work, but undefined don't
Also decoding the storage of such a contract always return null

See this commit for a summary of an example of the current behavior
9277430

Using null to represent the Unit value is ambiguous as it can have different meaning.

One solution would be to use a global constant ex: Taquito.Unit which would be required while encoding and returned while decoding storage which contains Unit type.

[jevonearth]

#248