Scheme list <--> JS array conversion and proxying

[jathak]

The old interpreter did this automatically whenever these objects passed between the two languages, which was a bad decision that lead to some hacks like:

; array lives in js land to avoid expensive conversion
(define sequence ((js "(arr)=>(type, a, b) => {\n"
  "if (type === 'add') {\n"
  "   arr.unshift(a); return;\n"
  "}\n"
  "if (type === 'dump') {a(true, arr); return;}\n"
  "return arr.slice("
    "Math.max(0, arr.length - a - b), "
    "Math.max(0, arr.length - a));\n"
"}\n") nil))

from lib/music.scm

Because of this, I made Scheme pairs and JS arrays pass to the other language with no conversion. JS arrays are readable in Scheme through code like (js-ref (js "[1, 2, 3]") "1"), but there's currently no good way to manipulate Scheme pairs from JS (due to dart2js making the properties unreadable).

There are two things I'd like to do:

• Add list->js-array and js-array->list procedures, or possibly piggy-back off vectors (Add vectors #23)
• Pass proxies to JS instead of the pairs themselves, to allow JS to access first and second directly.