try to preserve local function names for better stack trace

[bobzhang]

let g y =
  let h x = x + y 
  in  h

generated code:

function g(y) {
  return function (x) {
    return x + y | 0;
  };
}

it would better to have

function g(y) {
  return function h(x) {
    return x + y | 0;
  };
}

[stale]

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