feat(core): Implement mapExchange (replacing errorExchange)

[kitten]

Supersedes #1743

Summary

This avoids the breaking change in #1743 and finally replaces it with a complete mapExchange implementation that's similar but doesn't change the debugExchange.

The mapExchange is API-backwards compatible with the errorExchange and the errorExchange's export has been replaced with mapExchange.

The mapExchange exposes three options it accepts:

• onOperation: Allowing it to react to and replace operations
• onResult: Allowing it to react to and replace results
• onError: Allowing it to react to errors on results

The onError option is provided for backwards-compatibility with the errorExchange

The onOperation and onResult functions may respectively,

• return undefined | void which will cause them to not replace the incoming operations/results
• return new operations/results, which will replace the incoming ones, allowing it to remap them
• return promisified operations/results, which allows them to replace operations and results asynchronously, awaiting the promise they return

Set of changes

• Implement mapExchange
• Replace errorExchange export with mapExchange
• Update docs