Expose more of the Vec API on BufferVec

[james7132]

What problem does this solve or what need does it fill?

More efficient mutation APIs for BufferVec than just push, reserve, and clear. Repeatedly pushing large quantities can be quite inefficient due to needing to repeatedly do size and capacity checks on the underlying Vec.

What solution would you like?

Expose more of the mutation APIs to more efficiently build BufferVecs. Most notably append for grouping chains of Vecs together.

What alternative(s) have you considered?

• Repeatedly pushing the items onto the BufferVec.
• DerefMut on the underlying Vec.

Additional context

This would both simplify and speed up the "prepare" stage of rendering of entities with large quantities of mutable values, including but not limited to particle systems and large scale batching.

[Jdogstar]

Out of curiosity, besides append, what other API's would you consider effective to add?

[james7132]

#6833 requires adding Vec::truncate to the API, Vec::append/Vec::extend are also other options worth considering as well.