Add PURE annotation to generated typedef

[skeggse]

Marking this statement as pure provides a workaround for #239 via live code inclusion/treeshaking and the disabling of function annotations. For example, a rollup configuration object like this will fully prune usage of flow-aws-lambda with annotate: false configured:

import babel from 'rollup-plugin-babel';

export default {
  input: './index.js',
  output: {
    file: './dist/index.js',
    format: 'cjs',
  },
  external: ['flow-aws-lambda'],
  treeshake: {
    pureExternalModules: true,
  },
  plugins: [
    babel({
      plugins: [['flow-runtime', { annotate: false }]],
      presets: ['@babel/preset-flow'],
    }),
  ],
};

[skeggse]

@gajus does this seem reasonable (not a comprehensive solution but not problematic)?

[gajus]

This solution seems fine. However, can you please add a test case (even though there are others that are failing).