Use node snapshots

[avaer]

Most of Exokit is implemented in Javascript, so each startup has compilation overhead -- this is particularly true for multiple parallel window objects, which each has its own startup phase.

Projects like Chromium and Electron use snapshots to speed up the startup. And luckily there is some work to get that feature into node as well: nodejs/node#17058

We probably want something very similar in Exokit.

Reading for Electron:

• https://flight-manual.atom.io/behind-atom/sections/how-atom-uses-chromium-snapshots/
• http://peterforgacs.github.io/2018/09/12/How-to-create-a-V8-snapshot-of-your-javascript-file/

[avaer]

Not the same thing as this issue, but for Android builds we have snapshots disabled.

Enabling that should give a reality tabs startup perf boost, and is a prerequisite for this issue.

[avaer]

Snapshots for V8 (not yet node) are enabled on the mobile builds now.

[avaer]

node v12.5.0 supports snapshots for its library code now.