[#67] Add specialized multisig contract

Description

This PR copies the specialized multisig contract from the https://github.com/tqtezos/lorentz-contract-multisig repo to morly-multisig package.

The tests for the specialized contract was made by copying the tests from generic multisig contract and making necessary changes to work with the it.

EDIT: The contract has been slightly amended to include key count/threshold check.

Related issue(s)

Resolves #67 (closed)

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