New style lint: Warn for non_camel_case_types of ty capture in macro_rules

[tesuji]

For example:

macro_rules! foo {
    ($int_ty:ty) => {
//    ^^^^^^ help: convert the identifier to upper camel case: `IntTy`
        <$int_ty>::max_value()
    };
}

const BAR: u32 = foo!(u32);

[flip1995]

I'd suggest to implement this as a pedantic lint, since using camel case for all macro variables is pretty common and not really specified by Rust style guidelines.