Partial support for duplicated symbols

According to The Big O of Code Reviews, this is a O(n) change.

This PR partially fixes #68:

it re-exports each named crate as default export.
it documents working with megazords.
including the limitations of the Swift backend.

The second part to fix #68 is a little more involved, but is not yet worth it:

for imported types and their converters, the bindings should use a more fully qualified name. e.g.

Instead of:

import { Crate1 } from "./crate1";
import crate1 from "./crate1";

const { FfiConverterTypeCrate1 } = crate1.converters;

// In use
type MyType = {
  scalar: Crate1,
  optional: Crate1 | undefined,
  list: Array<Crate1>,
}

// FfiConverterTypeCrate1 being used.
const lift = FfiConverterTypeCrate1.lift.bind(FfiConverterTypeCrate1);

The next part needs to generate code:

import * as crate1 from "./crate1";

// In use
type MyType = {
  scalar: crate1.Crate1,
  optional: crate1.Crate1 | undefined,
  list: Array<crate1.Crate1>,
}

// FfiConverterTypeCrate1 being used.
const lift = crate1.converters.FfiConverterTypeCrate1.lift.bind(crate1.converters.FfiConverterTypeCrate1);

I think I will file a new issue with this in, and close #68.

jhugman / uniffi-bindgen-react-native

Partial support for duplicated symbols #143