Allow spaces right before/after tag parentheses

[dahlia]

This patch fixes #69.

The parser previously has a bug disallowing any whitespaces (including newline) right before and right after union tags' parentheses. For example, the previous parser doesn't accept the following union declaration:

union shape
    = circle (  // ⬅️ Newline/whitespaces not allowed ➡️
        point origin,
        offset radius,
        // ⬅️ Newline/whitespaces not allowed ➡️
      )
    | rectangle (  // ⬅️ Newline/whitespaces not allowed ➡️
        point upper-left,
        point lower-right,
        // ⬅️ Newline/whitespaces not allowed ➡️
    )
    | none
    ;

[Kroisse]

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