Check attribute multiplicity

[ironcev]

Currently, attributes that should be applied only once per item can be applied an arbitrary number of times. E.g., this compiles without any errors or warnings:

#[inline(always)]
#[inline(never)]
#[inline(always)]
#[inline(never)]
pub fn inlined_or_not() {
}

#[test]
#[test]
#[test]
fn test() {}