Liblet

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- A new Productions class has been introduced to allow a nicer HTML
representation of grammar productions.
- Added the Graph.neighbors method (to iterate on adjacency lists).
- Made Tree iterable (to allow for root and children unpakcing).
- Major code reformatting (now 2 spaces are used instead of 4).
- Cosmetic fixes (removing some stale imports and commited files).
- Now docs point to Gitpod (Binder is completely gone).

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- From binder to gitpod.
- Updated to the new graphviz api.
- Now sing GitHub Actions to run tests (instead of Travis).

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- Getting ready for the next year.
- Fixed unittest discover parameters.
- Updated dependency to ANTLR 4.9.3.

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- Emergency fix for LLVM toolchain detection code.

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- Added installation instructions related to the new LLVM module.
- Completely removed *beaconing*.
- Added support for tox testing (with Python 3.7, 3.8, and 3.9).

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- Added an experimental LLVM module to support IR code generation.
- AnnotatedTreeWalker.TREE_CATCHALL does not collect None subtrees.
- AnnotatedTreeWalker.register decorator trims ending _ (to allow builtin named
keys).
- AnnotatedTreeWalker.register optionally takes the function name.
- Annotated trees now have an attr attribute defined as a AttrDict wrapping the
root dict.
- Added AttrDict in liblet.utils.
- ANTLR.context and ANTLR.tree accept a antlr_hook parameter that allows
complete configuration of the underlying lexer and parser.
- Stack and Queue now support reversed iteration.