Some flat_serialize tests fail under Miri

[syvb]

Two tests in our flat_serialize crate fail when run under Miri, a Rust interpreter that can detect some undefined behavior:

$ cargo miri test --target x86_64-unknown-linux-gnu
...
failures:

---- tests::many_enum stdout ----
thread 'tests::many_enum' panicked at 'called `Result::unwrap()` on an `Err` value: InvalidTag(0)', crates/flat_serialize/flat_serialize/src/lib.rs:1317:83

---- tests::nested_slice stdout ----
thread 'tests::nested_slice' panicked at 'called `Result::unwrap()` on an `Err` value: NotEnoughBytes(38)', crates/flat_serialize/flat_serialize/src/lib.rs:1065:51


failures:
    tests::many_enum
    tests::nested_slice

test result: FAILED. 18 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out

I don't know why this only happens with Miri, and not when run normally. It seems flat_serialize is doing memory reads in a way that is potentially UB.