Make further bundle size optimisations

[kitten]

This accomplishes a couple of things.

Firstly it sets up Rollup to compile process.env.NODE_ENV to be "production" for our minified bundles so those accurately reflect what urql would compile to in a production environment.

Then it adds some initial bundle size optimisations:

• Remove rest spread from Query/Subscription components as it's not needed
• Replace debugExchange with a noop in production
• Remove usage of addMetadata with latency times from fetchExchange in production
• Remove fallback and fetch error for unknown operations in production
• Extract response/error parsing from fetchExchange and subscriptionExchange into a common file

This appears to currently save us about 0.3kB (4.62 kB => 4.32kB) min+gzip

[JoviDeCroock]

I think this in combination with the removal of graphql-tag will drop the bundle size on phobia down by quite a good margin. I think it's ready personally don't see that much aspects to make it smaller atm (apart from things that reduce readability). We could also mangle really internal properties