Proposal: use ref(() => Map) instead of ref('Map')

This is a:

- [ ] Bug Report
- [ ] Feature Request
- [x] Question
- [ ] Other

Which concerns:

- [x] flow-runtime
- [x] babel-plugin-flow-runtime
- [ ] flow-runtime-validators
- [ ] flow-runtime-mobx
- [ ] flow-config-parser
- [ ] The documentation website

---

### Using`ref` with the name of a type as a string is fundamentally problematic

Because the babel module transform mangles the names of imports. It has to because it can't know at compile time if the value of an export will change, because of circular imports, etc.

So any code that `babel-plugin-flow-runtime` converts to `ref('Map')` will break if `Map` comes from `import Map from '...'` because that import will get transpiled to
```
var _map = require('...')

var _map2 = _iteropRequireDefault(_map)
```
(see #177)

I think the best way to get around this would be to use a function instead of a string: `ref(() => Map)`. That way it can still be evaluated at runtime, but babel's module transform will convert that to `ref(() => _map2.default)`, so it won't break.

@phpnode thoughts?