Liblet

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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.

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- The liblet.display.Tree.with_threads method puts nodes at the same rank according to
their depth in the tree.
- The liblet.antlr.ANTLR.tree method ignores <EOF> tokens for non simple trees.