Feature: Put a promise queue around file reading.

In bundling, we read all files into memory (and stat them). This can be a limitation we don't necessarily need. Our actual constraints:

- Read files in order.
- As file contents become available, do `zip.append` in sorted order.
- Wait on any file contents before further parallel reading until we finish `zip.append` to `n` concurrency.

See: `p-limit`, `p-queue`, etc.