Liblet

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- Introduced CYKTable (and deprecated cyk2table).

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- Productions now sorts the table rows (in production order).
- Updated examples to use Productions instead prods2table (now deprecated).
- Testing coverage is reported using Coveralls (instead of Codecov).

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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.