This adds support for `Expr` nodes in `condition` fields and
`SwitchCaseOp.target`.  The general `c_if` condition checking is lifted
into an `Expr` node as part of the OQ3 export, so the form of them
automatically becomes like the new-style outputs, depending on how
`expr.lift_legacy_condition` chooses to raise them to `Expr` nodes.

As part of the refactor of the `Expression` subtree of the temporary
AST, I removed `SubroutineCall`.  Its definition was severely
out-of-date (a form from before even the initial arXiv draft), where it
was statement-like instead of expression-like.  There is currently no
support in Qiskit for representing these sorts of subroutines properly,
and the OQ3 exporter would produce invalid OQ3 if it encountered
something that it thought should be a a subroutine call.  This commit
fixes that bug, correctly raising an exception that non-unitary
instructions are not yet supported for export.

This commit also removes the special casing from the `CXGate` into the
exporter itself; it was more convenient in the new structure of some
internal components, and it's a data model we don't want to support
anyway.

This is what the QSS stack expects (they don't like bitstring literals
in expressions yet), and is more in line with what we used to do anyway.