Adverserial use of `make_bitflags!` macro can cause undefined behavior

The macro relied on an expression of the form Enum::Variant always being a variant of the enum. However, it may also be an associated integer constant, in which case there's no guarantee that the value of said constant consists only of bits valid for this bitflag type.

Thus, code like this could create an invalid BitFlags<Test>, which would cause iterating over it to trigger undefined behavior. As the debug formatter internally iterates over the value, it is also affected.

use enumflags2::{bitflags, make_bitflags};

#[bitflags]
#[repr(u8)]
#[derive(Copy, Clone, Debug)]
enum Test {
    A = 1,
    B = 2,
}

impl Test {
    const C: u8 = 69;
}

fn main() {
    let x = make_bitflags!(Test::{C});
    // printing or iterating over x is UB
}