[#124] revised Indigo operators

Description

Problem: some operators in Indigo are named in an odd way (usually with a .) to avoid conflicts with Haskell operators, additionally many of them operating on structures have been added organically and do not follow any convention, being confusing. Moreover, many of them do not have a prefix counterpart, which may be useful to have available.

Solution: rename operators, replacing the ones from Prelude in case of name clashing and based on a convention in case of structure-based. Additionally, add all necessary prefix counterpart using the names of Michelson instructions where one exists.

Note: tests have been split between Indigo code and tests-specific because (especially with the additional changes) there are too many naming conflicts.

Related issue(s)

Resolves #124 (closed)

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