[#342] Implement branchout/offshoot in Nettest

Description

In our effort to improve the Nettest interface and catch up with the Integrational engine, we're adding branchout and offshoot to Nettest.

Supporting these features on a real network (via morley-client) would require some deep changes, so for now we'll only support them in an emulated environment (i.e. Michelson.Runtime). We added an additional capability EmulatedImpl that contains methods that can only be run in an emulated env.

Related issue(s)

Resolves #342 (closed)

Resolves part of #53 (closed)

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