Unintuitive operator precedence

[llogiq]

Warn on instances of _ << _ + _ (or similar cases involving seldom-used operators). While those are not wrong, they can become footguns for anyone trying to reason about the code.

Suggest parenthesizing the operation.

[llogiq]

I have a few examples on http://is.gd/Rz0BtT (rust playpen).

The lint should detect ExprBinary(op1, ExprBinary(op2, …), …) or ExprBinary(op1, …, ExprBinary(op2, …) where op1 and op2 are from different operator classes (arithmetic vs. bit-shifting/bit-combining).

[llogiq]

Done.