rust: fix clippy 1.85 precedence warnings

rust: fix clippy 1.85 precedence warnings

warning: operator precedence can trip the unwary
   --> src/jsonbuilder.rs:781:36
    |
781 |                 buf[offset] = HEX[(x >> 4 & 0xf) as usize];
    |                                    ^^^^^^^^^^^^ help: consider parenthesizing your expression: `(x >> 4) & 0xf`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#precedence
    = note: `#[warn(clippy::precedence)]` on by default