Liblet

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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).

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- Bumped ANTLR jar to version 4.8.
- Fixed a few typos in docs (thanks also to andreazube)

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- Bumped the major version, non new features expected since this year class is done.
- Added the parameter fail_on_error to the liblet.antlr.ANTLR.context method to make
it return None in case of parsing errors
- Modified the liblet.antlr.ANTLR.tree method so that it returns None in case of
parse errors
- The liblet.antlr.ANTLR.tree method does not include nodes for the EOF token.