Using Debian gir files

It does not work well with possibly errors during iteration and
requiring to possibly resync.

No need for a custom enum

This function is especially usefull when using the leaks tracer. It was removed
in commit e7a0543c.

Serialization and deserialization are asymetrical: ser uses string literals
and de uses a dedicated `struct`, leading to possible inconsistencies.

The idea is to rely on a single `struct` for both ser and de. The only
problem is that, in order to avoid unneeded copies, `buffer` requires an owned
`ByteBuf` for de and a borrowed `Bytes` for ser. `Either` could help adapting
to both situations.

This allows making sure serialization and deserialization are in sync for
types with asymetrical implementations.

`Caps` can be `any`, `empty` or matching a list of `Structures`.

... and use the same `struct` for serialization and deserialization so
that fields mapping consistency is ensured at compilation time.

Note: clippy didn't raise any warning

`CapsFeature` is not available in `gstreamer-rs` yet. Use a placeholder
with a `None` value for now.

Use a `DeserializeSeed` to pass the newly created and named `Structure`.

+ fix erroneous `datetime` instead of `buffer` in `BufferRef::serialize`
+ remove ser/de for `Value`s with types `ULong` & `ILong`