[#774] Add typechecking and better errors for sapling

Description

Problem: There is no support for the sapling feature, which results in poor user experience, particularly because parsing a contract with sapling types will result in an error that's very unclear.

Solution: Add some supports for sapling (typechecking and better error messages), update the tests.

Related issue(s)

Resolves #774 (closed)

✅ Checklist for your Merge Request

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• 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.

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    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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