RFC (Graphcache): Change SerializedEntries to provide Map to storage adapters

Summary

Change SerializedEntries to return a Map instead of an object.

This will prevent react-native clients using Hermes from running up against 196607 properties limit and breaking caching. https://github.com/facebook/hermes/issues/851

Proposed Solution

Change the type of SerializedEntries to SerializedEntries = Map<string, string | undefined>;

Downsides

• SerializedEntries can no longer be JSON.strinigify'ed without using the replacement function
• Breaking change for all storage adapters (including both shipped as a part of urql)
• No possibility of ES5 support