[TM-140] Support unpacking from `Readable`

Description

Problem: We are able only to unpack values from Optimized representation, which is inconvenient when we unpacking things like addresses or public keys, because they optionally can have Readable representation.

Solution: Add function to unpack from Readable representation to Michelson value.

We need such functionality in one of our other projects.

Related issue(s)

https://issues.serokell.io/issue/TM-140

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