fvl_to_letchain sucks

[jpolitz]

js_to_exprjs currently uses specially-named identifiers to transform surface JS into an intermediate form (ExprJS). The transformation looks like:

function f(x) {
  var y = 5;
}

f = func(this, args) {
  let %%x = undefined
  let %%y = undefined
  %%y = 5
}}

Then the next step looks for %% variables and turns them into context objects. We should have a special let form that binds JS-ids specifically, and not use the %%-hack.