Security Vulnerabilities: flatbuffers: read_scalar and read_scalar_at allow transmuting values without unsafe blocks

[BohuTANG]

Describe the bug

https://deps.rs/repo/github/apache/arrow-rs#vulnerabilities

flatbuffers: read_scalar and read_scalar_at allow transmuting values without unsafe blocks
RUSTSEC-2020-0009

The read_scalar and read_scalar_at functions are unsound because they allow transmuting values without unsafe blocks.

The following example shows how to create a dangling reference:

fn main() {
    #[derive(Copy, Clone, PartialEq, Debug)]
    struct S(&'static str);
    impl flatbuffers::EndianScalar for S {
        fn to_little_endian(self) -> Self { self }
        fn from_little_endian(self) -> Self { self }
    }
    println!("{:?}", flatbuffers::read_scalar::<S>(&[1; std::mem::size_of::<S>()]));
}

To Reproduce

https://deps.rs/repo/github/apache/arrow-rs#vulnerabilities

Expected behavior
A clear and concise description of what you expected to happen.

Additional context
See advisory page for additional details.