Tri.struct

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* Improvement: If a Struct subclass implements the `__missing__` method,
it will not be called before GetAttr on attribute access, but will be
called before GetAttr on dict access.

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* Bugfix: Clear KeyError in CStruct getattr before trying GetAttr,
otherwise the KeyError may "leak out". Also, only clear the error
and attempt GetAttr if the original error was a KeyError.

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* Add mixin class `Frozen` to make read-only versions of a dict-derived
class (typically a Struct or a subclass there of.)

* Use the `Frozen` mixin to implement `FrozenStruct`

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* Add keyword arguments to `merged` function.

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* Redo the C implementation to be a "heaptype", and remove the hack of
setting `__dict__` = `self`. Instead, `object` will control the type
storage, letting us "insert" attributes into the object without
polluting the `dict`.

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* slim down interface to again match dict
* add tri.struct.merged function to join structs
* add optional C implementation to speed up instantiation