from_bits accepts non existing flags

[arturoc]

    #[test]
    fn test_from_bits_edge_cases() {
        bitflags! {
            struct Flags: u8 {
                const A = 0b00000001;
                const BC = 0b00000110;
            }
        }


        let flags = Flags::from_bits(0b00000100);
        assert!(flags.is_none());
    }

Unless I'm missing something this test should pass but it fails cause from_bits accepts flags that are not declared.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6fd4adbddc8b8740cbd35af2306073ca

This is related to this issue in the implementation of iterators in this PR #204 (comment) Using from_bits instead of from_bits_unchecked should allow to produce any valid flags that are not a combination of other flags but at the moment from_bits seems to accept any flag that is included in a combination even if it's not declared. I can try to send a PR.