Move (some) not-yet-implemented errors to later stages

[sbillig]

What is wrong?

The analyzer rejects code that uses features that aren't yet implemented in some stage of the compiler pipeline. In some cases, it would be convenient to implement and test support for those features in the earlier stages (parser/analyzer/lowerer) before they're supported in the late stages. Eg the tests for lowering of nested tuples (tuples in tuples, or tuples in arrays) (#459) can't analyze the lowered AST because the lowered code uses nested structs.

How can it be fixed

Let the late stages emit their not-yet-implemented errors where appropriate.