type stability and compile time improvements

[rafaqz]

A grab bag of changes to improve compile time:

• type stable fields in e.g. Sync and Async, use of empty strings instead of nothing
• explicitly typed Dict
• less use of the js"" macro
• function barriers where there is type instability

[rafaqz]

It doesn't break Interact.jl or Blink.jl or anything they use, they're the motivation for this. Maybe other things, I'm not sure...

The other PR is more important than this too, this is comparatively minor. If you want to see benchmarks for this, we should merge that and rebase and compare to that base line.

After that and the other improvements to InteractBase/Widgets.jl/AssetRegistry.jl, This PR gives another 20% improvement or so getting slider under one second. Nothing sensational, but this is death by 1000 cuts... it takes all 5 PRs to get slider from 15s to 1s...

[halleysfifthinc]

Bump. Could we get this rolling? With #476 / #478, I think there are substantial enough changes to warrant a minor release, which would be breaking since WebIO is pre v1. Backwards compatibility wouldn't need to be guaranteed in that case.

[rafaqz]

I lost some steam on this after fixing Parsers.jl, which was the major source of compile time here. But yes we should clean it up.