Liblet

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- InstantaneousDescription has been subclassed in two specilized classes:
TopDownInstantaneousDescription and BottomUpInstantaneousDescription.
- Stack and Queues copy method has been deprecated in favor of more
standard use of the copy module (the shallow copy obtains a separate
storage by overriding __copy__).
- InstantaneousDescription and its subclasses shallow copies don't share
the stack with the original object (obtained by overriding __copy__).
- Improved deprecations (now the warning module is used instead of raise).

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- Added a first version of the InstantaneousDescription class (in liblet.automata).
- Documentation improvement (substituted :obj: with :class: where appropriate).
- Bumped ANTLR version to 4.9.1.
- Created FUNDING.yml for GitHub sponsorship.

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- To avoid confusion with peek of Stack and Queue, utils.peek has been
deprecated in favor of utils.first.
- Updated Automaton and StateTransitionGraph to allow representing item
set with frozenset instead of tuples.
- Modified utils.closure so that it always return the computed value,
in this way if just one iteration is performed, the returned type
coincides nonetheless with the type returned by the wrapped function.
- Fixing some graphviz visualization sizes.

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- Added a missing import.

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- Bumped to beta.
- Added display.animate_derivation (and hence ipywidgets to the deps).
- Dderivation now admits productions and not only indexes.
- Added utils.uc to append unicode combination chars.
- Grammar.restrict_to now makes explicit that the start symbol must be kept and
throws away productions containing also not specified terminals.

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- Now Derivation accepts also an iterable of steps (both for left/rightmost and
general case).