Liblet

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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.

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- Added new documentation for recently added classes and a few utils.
- Moved the liblet.display.warn method to liblet.utils.warn.
- Fixing the None node display style in liblet.display.Tree.with_threads.

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- Added HTML escaping to liblet.dislpay.iter2table and liblet.dislpay.dict2table.
- Added liblet.display.Tree.with_thread method.
- Added liblet.antlr.AnnotatedTreeWalker class.

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- Added and fixed ANTLR class tests.

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- Added documentation for the Tree class.
- Refactored heavily the ANTLR class.
- Added documentation for the ANTLR class.

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- The display.Tree class uses pseudo-HTML Graphviz labels if the root is a dict.
- The antlr.ANTLR class keeps track of the generated sources (in the sources field).