zvariant 3.15.0.

🚑️ Drop 64-bit offsets support on 32-bit systems. If the native pointer size is 32-bit, then the
64-bit offset support is not needed. This also fixes the assumption that when a 32-bit offset is
bumped, we end up with a 64-bit offset. This is not true on 32-bit systems because we use usize
for offsets, which is 32-bit on 32-bit systems. Fixes #357.
⚡️ impl From<Cow> for Str.
⚡️ Drop redundant inline. Inlining request only makes sense for very small functions.
🔥 Drop support for 128-bit offsets in gvariant. This was just cool to have thing but in reality will we really need support for memory larger than 16 hexabytes? I don't think so. Not anytimne soon at least. If we do, it'd be easy to add this back.
🔥 Drop manual Default impl of EncodingFormat. After Rust 1.64, we can use the derive for this.
⬆️ Update enumflags2.
✅ Don't assume native endianness in tests. Glib always uses native endian format so we should use native endian too if we want to decode from it to test compatibility. This fixes the number-related tests on big-endian systems. Fixes #358.
♻️ Refactor some test code. Unify some value equality assertions.
⬆️ Bump our MSRV. More and more dependencies are requiring Rust 1.64.0, so let's bump our MSRV to match.
🎨 code comments should also adhere to 100 character limit.
🎨 2 tiny formatting fixes.
🎨 Make use of bool::then_some.