Note: contains 1000 lines of TCL tests generated by cursor :] runtime
changes are smaller and this actually deletes code in aggregate.
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Main change is to support arbitrary expressions in RETURNING instead of
a specialcased subset, but also e.g. disallow returning TABLE.* which is
illegal syntax in SQLite.
Main idea is we add the columns of the target table (the table affected
by INSERT/UPDATE/DELETE) into `expr_to_reg_cache`) and then just
translate the RETURNING expressions as normal
Closes#3942
fca32737e7