[#836] Typechecker fixes

Description

• Remove unused TypeCheckPack mode

• Do not throw error in the typechecker

• Set typecheck mode in typeCheckValue

  Apparently, SELF inside a pushed lambda unit unit was considered well-typed by morley 😕 Well, not any more.

• Hotfix: give IsNotInView in test typecheck

  Follow-up to #829 (closed), morley repl complains about views when typechecking SELF, TRANSFER_TOKENS, etc, which is confusing and wrong.

• Add a contract testing self in lambda

  Apparently we didn't test for it? Well, here's a test.

Related issue(s)

Resolves #836 (closed)

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