Unify name mangling rules

Since BuckleScript is going to release a v8 and maybe Reason a v4, I think now could be a good time to fix name mangling issues once for all and stick to one rule.

Right now we have this Reason code:

let foo = {"_to": 10, "_type": 1};
type bar = {
  _to: int,
  _type: string,
};
let bar = {_to: 10, _type: "bar"};
ReactDOMRe.renderToElementWithId(<input type_="text" />, "preview");

compiling to:

var foo = {
  to: 10,
  type: 1
};
var bar = {
  _to: 10,
  _type: "bar"
};
ReactDOMRe.renderToElementWithId(React.createElement("input", {
          type: "text"
        }), "preview");

which is definitely not coherent.

I'd be in favor of using trailing underscores since leading underscores is already used to name unused variables but I'd be happy with any coherent rule.

Start by comparing the Reason examples in the issue with their generated JavaScript, focusing on object keys, record fields, and JSX attributes. The issue does not specify a final naming rule, so completion depends on agreeing on one coherent rule and applying it consistently across these cases.