fix(index.html,README): async changes / fix(compiler): only resolve imports once / refactor(compiler): split out parsing

[bryangingechen]

Here are a few preparatory changes I made while working on the upcoming ES module PR (I hope to push that either later today or tomorrow). In hindsight, this could probably be three separate PRs, sorry!

• The changes in index.html (db2e4e9) and README.md (ac97902) are fixes related to async changes in #11 and #12. In the README, I just added await in a bunch of places where it looked like it was now necessary. I didn't test the snippets separately though, so it'd be good if you could make sure I didn't screw up there.

• There are a few changes to compiler.js (they're unfortunately squished into one commit ac97902):

  • From lines 1-173 (in the new version), I did a slight refactor to split out some parsing code from the code which creates the define function. The new functions will be used in the upcoming ES module compilation PR.
  • In lines 181 and 183 and 263-264, I changed some .map() calls which didn't return anything to for ... of loops.
  • In lines 240-253 I changed things so that imported modules are only resolved once.

[bryangingechen]

Not sure what's going on with the github actions build...

[bryangingechen]

~In hindsight, 6c1e0ed doesn't make much sense. I'll force push with something better shortly.~ done

[bryangingechen]

Closing in favor of #15 (I was hoping that doing this in different PRs would make it easier to review, but I found a bug in 6e140ee, which I fixed in 87dd806).