Fix message generation distribution with '|'

[Dirakon]

Currently, the message generation distribution is fair, unless:

1. the first expression in OneOf series is OneOf in parentheses
   ("1" | "2") | "3" | ... will likely show unexpected results
2. the last expression in OneOf series is OneOf in parentheses
   "1" | "2" | ("3" | "4") will likely show unexpected results

Proposed solution: make parentheses into a special expression type and analyze them accordingly.