Avoid recomputation when handling let patterns

[jmackie]

Need to do something similar to #114 only for let bindings.

For example:

module Example exports (..);


type T = T(Int);

some_expensive_function = fn (i) -> T(i);

fixme =
    let
        T(i) = some_expensive_function(5000);
    in
    i;

Generates:

function T($0) {
  return ["T", $0];
}
function some_expensive_function(i) {
  return T(i);
}
const fixme = (() => {
  if (some_expensive_function(5000)[0] === "T") {
    const i = some_expensive_function(5000)[1];  // ISSUE: some_expensive_function called again!
    return i;
  }
  throw new Error("Pattern match error");
})();
export { fixme, some_expensive_function, T };