Integrate zephyr dead code elimination

[andys8]

Hi,

I wanted to see how much effect zephyr dead code elimination would have on a production build. It shaved of ~ 80 kb (gzipped) / 535 kb (zipped) / 50+ % (if I'm not mistaken).

So I'm sold on trying to integrate it in the build. Here is a draft PR, that shows the current integration.

If you want to give it a try, there is a pre-compiled branch and you can use it like this in package.json:

    "purs-loader": "andys8/purs-loader#poc/zephyr-build",

Discussion

I've still open questions. That's why this is a draft.

• The README says there is already dead code elimination. Can you provide details what is meant?
• This needs to be integrated into bundle. I'm not using bundle so far, because it will fail resolving TypeScript/Assets for me. It should be done. I can't test it.
• What details are missing for documentation?
• I'm currently re-using the configured output directory both for the compile and zephyr step. This avoids created a temporary folder. It seems to work fine for me. Could this lead to issues?

Issues

Closes #143

[toastal]

I opened a PR to this branch moving package.json's main to "src/index.js". This allowed me to use this fork via git+https, and by using this zephyr option, it cut the size of my output by about 50%.

[toastal]

Usually zephyr's output goes to dce-output. I'm not sure if about your last comment either though.

[toastal]

I did run across a big issue with this recently though: you can only have one entry point. Zephyr allows you can pass in multiple files to make sure you don't dead-code-eliminate code for your other entries. Unless I missed something, this fork doesn't collect all of the imported entries and then run the DCE. I decided to drop the module all together and require() from the appropriate /output/ or /dce-output/ folder per process.env.NODE_ENV (similar to how many projects suggest using with Parcel).