[#147] add stored entrypoint machinery in Lorentz for lazy code processing

Description

Putting all the code in the code section of smart contracts can cause them to be expensive, because it requires to parse it fully (and do some processing) even tho often you only need to use one of its entrypoints.

This is supposed to make it fairly easy and convenient to develop and use entrypoints as lambda stored in a big_map. Since big_map are lazy, this allows to process only the code that you are going to use.

Related issue(s)

Part of #147 (moved)

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