[#340] Specialize ContractRegistry for the case of a single contract

Description

Problem: Often there is only one contract in which case this interface is too complicated: you have to pass redundant contract name to each command. It could be simplified to having print, document, analyze and storage commands that don't take the contract name.

Solution: ContractRegistry now adapts when there is only a single contract. Each commands will not require inputing the contract name, including the storage command.

Related issue(s)

Resolves #340 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.