Fix sizeof precedence

[philipturnbull]

Increase the precedence of sizeof. Previously, an expression like:

len = sizeof(int) + 1;

would be parsed as:

len = sizeof ((int) + 1);

i.e the size of +1 cast to an int.

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Also:

• I also added tests to offsetof(...) to check that it behaves correctly.
• Wrong precedences for sizeof and conditional operator #51 mentions problems with the precedence of the ternary operator, but that appears to have been fixed since the issue was opened. I added a couple of tests to prevent regressions in the future.

Fixes #51

[amaanq]

Can you regenerate w/ the Rust cli crate on master?