typeck: Do high-level structural/signature checks before function body checks.
This avoids various ICEs, e.g. premature calls to cat_expr that yield the dreaded "cat_expr Errd" ICE.
However, it also means that some early error feedback is now not provided. This may be for the best, because the error feedback were were providing in some of those cases were false positives -- it was spurious feedback and a distraction from the real problem.
So it is not 100% clear whether we actually want to put this change in or not. I think its a net win, but others might disagree.
(Kudos to @arielb1 for suggesting this modification.)
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