The previous naming scheme used the name of the resulting bundle file.
However, there are cases where multiple bundles have the same filename.
This meant whichever bundle finishes last overwrites the previous ones
with the same name.
The updated naming scheme is `bundle-sizes-<CI_NODE_INDEX>.json`.
Instead of generating a separate info file per bundle, it now creates
one per process.
* Write size info to separate file per bundle
`bundle-sizes.json` contains the combined size information for every
build. This makes it easier to store and process, but it prevents us
from parallelizing the build script, because each process would need to
write to the same file.
So I've updated the Rollup script to output individual files per build.
A downstream CI job consolidates them into a single file.
I have not parallelized the Rollup script yet. I'll do that next.
* Parallelize the build script
Uses CircleCI's `parallelism` config option to spin up multiple build
processes.
