[#24] add custom Prelude and Lorentz modules to Indigo

Description

All Indigo modules have to import Prelude and Lorentz with a list of hiding rules. This is bothersome and a source of boilerplate. Additionally, this is also necessary for contract that are written using Indigo.

This MR adds Indigo.Prelude and Indigo.Lorentz modules, containing the necessary hiding rules. These are also included in the Indigo module so that contracts written using Indigo are now able to import Indigo without further changes.

NOTE: both Lorentz's Macros and Instruction have not been re-exported because many of them have an Indigo homonym and by using a qualified import it is more clear that they are Lorentz's.

Also, const and var have been renamed as in !202 (merged) (see discussion).

Related issue(s)

Resolves #24 (closed)

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