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Add `Expr` support to QPY for conditions and targets (#10392)

* Add `Expr` support to QPY for conditions and targets

This adds support to QPY for the current `Expr` nodes, for *all*
instruction parameters and conditions.  The `Expr` tree is written out
to the file in a sort of forwards Polish notation; each node has a type
code and header, followed by a type-code-specific number of `Expr`
children.

While only `IfElseOp.condition`, `WhileLoopOp.condition` and
`SwitchCaseOp.target` are allowed to have these nodes in Terra's data
model right now, the QPY serialisation does not need to have this
arbitrary restriction, and it's much easier just to write the general
case.

The backwards-compatibility guarantees of QPY are now brought to bear on
the `Unary.Op` and `Binary.Op` enumeration values. They were already
marked in the source code as their values needing to be part of the
stable public interface, and their use in things like QPY is the reason
why.

* Improve documentation of `EXPRESSION` payload

* Factor out magic numbers from discriminator sizes

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