test the combinator table as documentation

doc/choosing_a_combinator.md

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+# List of parsers and combinators
+
+## Basic elements
+Those are used to recognize the lowest level elements of your grammar, like, "here is a dot", or "here is an big endian integer".
+
+| usage | input | output | comment |
+|--|--|--|--|
+| `char!('a')` |  `"abc"` | `Ok(("bc", 'a'))`| matches one character (works with non ASCII chars too) | 
+| ` is_a!("ab")` |  `"ababc"` | `Ok(("c", "abab"))`|matches a sequence of any of the characters passed as arguments. `is_a_s` is an alias for `is_a`|
+| `is_not!("cd")` |  `"ababc"` | `Ok(("c", "abab"))`|matches a sequence of none of the characters passed as arguments|
+| `one_of!("abc")` |  ``"abc"`` | `Ok(("bc", 'a'))`|matches one of the provided characters (works with non ASCII characters too)|
+