Accept standard EBNF

[apalala]

There are many grammars and tools out there using the same variation of EBNF, and TatSu should be able to parse it.

Examples:

• https://www.bottlecaps.de/rr/ui
• https://github.com/python/cpython/blob/master/Grammar/Grammar

Changes required:

• Accepting :=, :=, and : for introducing the right hand side of rules
• Accept ()* and ()+ for closures
• Not require a ; to end a rule (indented expressions belong to the rule)
• Accept /* ... */ comments

[mgrazebrook]

I'd like this! Every time I come back to Tatsu, I find myself instinctively using : instead of = for rules.