Limit spans in bytes!() error messages to the argument in question

This constrains the span to the appropriate argument, so you know which
one caused the problem. Instead of

    foo.rs:2:4: 2:21 error: Too large integer literal in bytes!
    foo.rs:2    bytes!(1, 256, 2)
                ^~~~~~~~~~~~~~~~~

it will say

    foo.rs:2:14 2:17 error: Too large integer literal in bytes!
    foo.rs:2    bytes!(1, 256, 2)
                          ^~~