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Transpile equality if-chains as switches (#765)

We understand the chains of ``if``, ``elif`` and ``else`` in the bodies
of transpilable functions as switches, provided all the conditions
compare the same subject against constants, in one of the forms
``subject == label``, ``subject == label1 or subject == label2``, or
``subject in (label1, label2)``. The labels are string or integer
literals, or enumeration literals. A sole ``pass`` denotes an empty
branch. Other ``if``'s are refused for now; we plan to introduce the
general if-statement later.

The parse tree gets a ``Switch`` node, and ``StatementUnion`` restricts
the bodies of the branches to statements. The type inference checks
the subject and the labels, and treats the branches as block scopes.
Since Python has only function-level scopes, we refuse the collisions
of variable names where the two would diverge so that the meta-model
code behaves the same in all the targets.

C#, Go and TypeScript switch natively. Java switches with the arrow
form, except on ``long``, and C++ switches except on strings; there, we
fall back to chains of ``if`` and ``else if``. Python keeps the chain,
as the SDK still supports Python 3.9 without ``match``.

Along the way, we fix the declarations of local variables which had
been broken: Java declared them with Python type names,
TypeScript did not declare them at all, and the optional and pointer
inferrers of C++, Java and Go refused them.

The new common meta-model ``switch_statements`` covers all the variants,
and is live-tested in all the targets.

742 of 844 new or added lines in 13 files covered. (87.91%)

38127 of 43407 relevant lines covered (87.84%)

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