Fixes #159
Fixes #158

video: Add support for getcaps(), negotiate(), src/sink_query/event() vfuncs in VideoEncoder/Decoder

See gstreamer#444

And ignore some common warnings we don't care about in general, while
also making clippy errors fail the build.

`for x in foo` depends on foo implementing IntoIterator.

std has a trait for converting things into Iterators. Make use of it.

Transposing the item type lets us be a std-compatible Iterator.

The iterator is automatically resynced when resuming iteration after
yielding Resync. This lets some combinators like `collect` and `find`
work properly.

The derived implementation is restricted to `where T: Debug`, but the
iterator doesn't actually contain a value of type T.

Tests run parallel in multiple threads. This makes the log_handler test
flaky because it may see log messages triggered by other threads. Make
the handler ignore all messages not in the category we care about.

Exposing the corresponding struct element in GstElement

... otherwise they can't be resolved in dependent crates compiled
for edition 2018.

See #217

Attention: these changes induce breaking changes for the serde
representation of some types.

Serialization of `Value`s used to rely on the `get` function
followed by an `unwrap`. This means that optional types couldn't
be serialized when they were `None`.

This commit distinguishes between the optional `Value` `Types` and
the ones that always return some value.

The `Value`s, `Structure` fields and `Tag`s with following types are
now represented as `Option`s:

- `Buffer`
- `DateTime`
- `Sample` (note: this type is used for `Tag` images)
- `String` (except for `Tag`s - see the comment in `tags_serde.rs`).

The representations for these `Type`s remain unchanged when they are
used in-place (not as part of a `Value`).

Fixes #215

This mostly wraps enums, flags, and constants, will wrap RTPBuffer
next

The reference to the pad we get passed is not guaranteed to be valid for
the whole scope of release_pad(). It might be the last reference as
owned by GstElement, and gst_element_remove_pad() would destroy it then.

This only returns Err on type mismatches. If the field does not exist or
None it returns None, otherwise the value.

StructureRef::get() returns Err also if the field does not exist.

This improves build times as it allows cargo to reuse more previous
build results when building crates separately.

Also remove the serde-pickle dependency. It was only used in a single
test and caused pulling in of quite a few dependencies.