Mangling exports

Bug: Babili mangles exports

Possible solutions:

```js
// case 1
// in
export const LongName=foo
// out
const a=foo; export {a as LongName}

// case 2 single reference - export statement
// in
const LongName=foo;export {LongName}
// out
const a=foo;export {a as LongName}

// also in case 2,
// DCE could have combined the declaration and the export statement
// as export declarations are statically analysed
// it doesn't matter where it is placed in the file (with ref.loc >= defn.loc)
// replacing const x = foo; with export const x = foo; is a safe replacement

const LongName=foo;export {LongName}
export const LongName=foo;

// case 3 referenced multiple times
// in
const LongName=foo;const Long2Name=()=>LongName(bar)+LongName(baz);export {LongName};export {Long2Name}
// out 1
const a=foo;const b=()=>a(bar)+a(baz);export {a as LongName};export {b as Long2Name}
// out 2
export const LongName=foo;export const Long2Name=()=>LongName(bar)+LongName(baz)
```

/cc @jdalton

Start by tracing Babili's export-mangling path using the three cases in the issue as behavioral examples. Verify how exported declarations, single-reference exports, dead-code elimination, and multiply referenced exports are handled; done means exported names remain stable while eligible local bindings are shortened without changing the examples' semantics.

Written by the indexing model from the issue text.