Export formats separately, without scoping

## Related issues

- ajv-validator/ajv-formats#68

By scoping formats into `fullFormats` and `fastFormats`:
- it's harder to do code generation as it's impossible to `import` a format in a single line
- it makes it more complicated not to have multiple imports of the same variable

As a breaking change proposal, let's export two modules instead:
- `formats-fast.js`
- `formats-full.js`

This way, when doing code generation, it'd be possible to have a single line import:
```js
import { int32 } from "ajv-formats/dist/formats-full"
```