IdStr represents UTF-8 immutable strings which perform optimizations for short
strings (< 16 bytes). The C type `GstIdStr` was introduced in GStreamer 1.26 as
a replacement for GQuarks.

This commit adds Rust bindings for the C type `GstIdStr`. Since this type will
be used in API which previously relied on GQuarks, the commit also adds a
compatibility implementation which can be used with GStreamer versions prior to
1.26, which is the first version to implement and use `GstIdStr`.

The crate [KString] was used as the inner implementation for the compatibility
version as it performs similar optimizations as `GstIdStr` and uses the same
threshold to trigger heap allocation.

See also: gstreamer!7432

[KString]: https://crates.io/crates/kstring

Part-of: <!1531>