Let's fuzz Bot.Expr.exprs to make sure that it has expected behaviour

[rexim]

introduced in #800

[rexim]

Rest of the work after #804:

• subcommand to generate default config
• expr normalization may generate less expressions than required by the config
• separate parser from Bot.Exprs (it should not depend on such things as Effect)
• stats of generated exprs
• fuzzing of trailing whitespaces between % and ,
• fuzzing of special characters in text nodes
• run fuzzer as part of the unit tests