[#567] Add methods for importing Lorentz `Contract`

Description

Add Contract that is convenient both for contracts coming from Lorentz and coming from .tz contracts, and methods for importing to it.

Related notes

There was a proposal to add a typeclass for converting anything to Contract (if I got it correctly?), I'm open to suggestions on how this should look like concretely. Currently, there is one Contract type, and all the functions of the testing framework work with it (except for those that are used in morley-test), and in my opinion that's just good.

Related issue(s)

Resolves #567 (closed)

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