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py2js: support rest parameters (*args) and spread arguments (#485)

* py2js: support rest parameters (*args) and spread arguments (f(*xs))

The py2js engine rejected `def f(*args)` ("rest parameters is not supported"),
and miscompiled `lambda *a: ...` by treating `*a` as an ordinary parameter, so
Python §3/§4 programs using them only ran on the CSE machine, PVML and wasm.

- A rest parameter compiles to a JS rest parameter: the array it collects is the
  Python list, as in CSE. The function is variadic (pyArity -1) with pyMinArgs
  set to its fixed-parameter count and a new pyRest flag, so arity() reports the
  fixed count like CSE and calling it with too few arguments is a TypeError
  ("f() takes at least N arguments but M were given").
- A `*xs` argument compiles to `...__py.spread(xs)` in both ordinary and tail
  calls, evaluated left to right with its neighbours. Like CSE, only a list can
  be spread (not a string); the callee gets a copy.
- A rest parameter that is not last (`def f(*a, b)`) is a compile error, as in
  PVML and wasm: the parser accepts it but there are no keyword-only parameters.
- A variadic user function crosses into a module with just its fixed parameters.

Chapters 1-2 still reject both via their validators.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* py2js: mark a rest function passed to a module as vararg

When `def f(a, *rest)` crossed into a module, closure_make was called without
isVararg, so the module's checked closure_call rejected any surplus argument
with InvalidArityError before pyClosureFunc could collect it into the rest
list. Pass `fn.pyRest` as isVararg, as the CSE and PVML bridges do, so the
declared arity acts as a minimum.

closure_make's isVararg argument is a GenericDataHandler extension that the
bare IDataHandler interface lacks, so pythonToModule, moduleToPython and
loadChunkImports now take the GenericDataHandler the engine already supplies.

Adds tests where a module calls a rest funct... (continued)

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