Fix use of "pseudo-syntax" in current lowering pass

The so-called "lowering" pass (really a kind of AST-to-AST legalization pass right now) needs to handle some basic scalarization of structured types, and it does this by inventing what I call "pseuo-expressions" and "pseudo-declarations."
For example, there is a pseudo-expression node type that represents a tuple of N other expressions, and certain operations act element-wise over such tuples.

The problem was that the implementation introduced these out-of-band expression/declaration types into the existing AST hierarchy which led to a dilemma:

- If these new AST nodes were declared like all the others (and integrated into the visitor dispatch approach, etc.) then every pass would need to deal with them even though they are meant to be a transient implementation detail of this one pass

- But if the new nodes *aren't* declared like the others, then they can't meaningfully interact with visitor dispatch, and will just crash the compiler if they somehow "leak" through to latter passes. And because they are just ordinary AST nodes from a C++ type-system perspective, such leaking is entirely possible (if not probable)

Hopefully that setup helps make the solution clear: instead of having the "lowering" pass map an expression to an expression, it needs to map an expression to a new data type (here called `LoweredExpr`) that can wrap *either* an ordinary expression (the common case) or one of the new out-of-band values. Any code that accepts a `LoweredExpr` needs to handle all the cases, or explicitly decide that it can't/won't deal with anything other than ordinary expressions.

Most of the code changes are straightforward at that point, although the whole "lowering" approach is a bit fiddly right now, so gertting the tests passing took a bit of attention. I'm not sure our test coverage of all this code is great, so I wouldn't be surprised if some failures are lurking still.

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