Followup changes to vellottie merge

#13 is quite large already, so this track issues and further enhancements to make after its merge.

Needed for release

• move util.rs into import.rs (from @DJMcNab)
• add lib.rs crate documentation (from @DJMcNab)
• reduce boilerplate in scenes/src/download/default_downloads.rs with a macro (from @DJMcNab)
• add usage to README
• print total file download at prompt time (from @DJMcNab)
• Introduce a changelog

Not needed for release

• Consolidate schema changes from LAC
• separate the with_winit example from vello into something that can be imported, to reduce copy/paste work
• include optional wgpu-profiler work from https://github.com/linebender/vello/pull/502 (from @DJMcNab) (superseded by the above)
• Non-linear interpolation with kurbo primitives (from @dfrg)
• Consolidate the runtime SplitValues((Value<f32>, Value<f32>)) into Value(Value<Point>) (from @dfrg)
• Pack tangent and hold frame into a u32 (from @dfrg) - See this discussion.
• Replace brush_with_alpha after https://github.com/linebender/peniko/issues/27

This is a tracking issue with several unchecked enhancements rather than one defined change; review the remaining checklist items and their linked discussions, including the discussion on packing tangent and hold frame. The mentioned files include import.rs, lib.rs, README, and scenes/src/download/default_downloads.rs, but no file or test is specified for the remaining work. Done requires completing a selected enhancement and updating its tests or documentation as appropriate.