Precedence17Expression → Precedence18Expression

to make room for operator-style expressions.

Done with the following command:

    find source -type f -exec sed -i 's Precedence17Expression Precedence18Expression g' {} +
Remove three outdated TODO comments

See 592fcfd.
Add OperatorStyleExpression, OperatorStyleMemberExpression

Includes some fallout that is mostly unavoidable when rebasing a
months-old branch onto latest master… in this case, Visitor was so
messed up that I just literally “sorted” it out by piping the visit()
methods into ‘sort -u’ with vim; that’s where some of the changes there
come from.
Add tests for OperatorStyle{,Member}Expression
Add OperatorStyleInvocation
Add tests for OperatorStyleInvocation