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Concrete events and messages share common attributes which can be factorized in generic builder `struct`s. This reduces noise in the concrete implementations.

Implement `deref` for concrete derivatives of `Event`, `Message` and `Query`. This allows accessing generic methods such as `is_sticky` for events, `get_seqnum` for messages or `is_serialized` for queries and `get_structure` for all.

Building an argument-less message such as eos yields an assertion failure due to the inner structure being null. The short term solution consists in checking that the inner `structure` is not `null` before attempting to insert `other_fields`. The consequence is that `others_fields` defined for argument-less messages will be ignored. A correction will be applied when GStreamer 1.14 is released thank to the introduction of `gst_message_writable_structure` (see https://bugzilla.gnome.org/show_bug.cgi?id=792928). Due to the dependency on GStreamer 1.14, the correction will be only available under the activation of a feature "v1_14".

Events are not affected as the build method the availability of `gst_event_writable_structure` and this function "will never return NULL". However, we can avoid a `structure` allocation for argument-less messages without `other_fields`.

The origin in protection events is optional, the stream id in
stream-start events is not.

Also return a segment reference instead of a segment copy when parsing a
segment event.

FormatValue is now renamed to GenericFormattedValue and the API slightly
changed. In addition there is now a FormattedValue trait, and a
SpecificFormattedValue trait plus types for Bytes, Buffers and the
existing ClockTime.

This allows to create functions like
  Pad::query_duration<F: SpecificFormattedValue>() -> Option<F>
and doesn't require the caller to unwrap the generic value anymore,
which is completely unneeded in these cases.

In addition, Segment became FormattedSegment<T> with API to
upcast/downcast between the specific formatted values and the generic
formatted value. This greatly simplifies usage of Segments.

We require types that can be sent between threads.

And also implement a FormatValue type that holds a value together with
its format to make usage of the positions/durations/seek positions/etc
more convenient and safe.

Fixes https://github.com/sdroege/gstreamer-rs/issues/3

https://github.com/rust-lang/rust/issues/44182

https://github.com/sdroege/gstreamer-rs/pull/22

Fixes https://github.com/sdroege/gstreamer-rs/issues/10

And make Message bindings more consistent