[#206] Split `Indigo.Prelude` into 2 modules

Description

Problem: Indigo.Prelude was originally used to define all the hiding rules in one place and then to be imported within Indigo. However, we also export as part of Indigo module which then creating conflicts due to we re-using the same function names and syntax to write Indigo contract.

Solution: Split current Indigo.Prelude into two. Indigo.Backend.Prelude which is used like it original purpose. New Indigo.Prelude which should be used as Export to write Indigo contract.

Related issue(s)

Resolves #206 (closed)

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