Improve performance by offering a `buffers()` method that returns `Buffer[]`

[dmichon-msft]

I recently discovered that invoking buffer() on a ConcatSource incurs a massive performance overhead by invoking Buffer.concat, particularly if there are layers of CachedSource -> ConcatSource -> CachedSource -> ConcatSource.

Invoking source() and the calling Buffer.from is considerably faster because it avoids the intermediary allocations.

We have writev nowadays, so there's no reason we couldn't keep the data as Buffer[] all the way until the final write to the file system and avoid repeated copying of memory during the concatenation layers.

[alexander-akait]

@dmichon-msft I see, do you want to send a PR (we can keep old buffer() and mark them as deprecated plus implement a new sourceBuffer(), maybe a better name?)? But honestly I don't think we have a lot of buffer() calls inside webpack (and official plugins), but non official can use it.

Also can we try to bench it using different size and nesting with source() and Buffer.from(), and buffer(), will be interesting to look at this