Resolves #1207

Summary

This updates the shared context object to be provided on a global ref, which is then used in various cache methods, via cache.keyOfEntity, to alias the parent argument, which can be found on resolvers and updaters, to info.parentKey. This is done by adding (publicly) info.parent, which is the current parent's data as it is. (This avoids us having to share it on a separate object)

This further refactors store.keyOfEntity to take care of all key normalisation that has been done repeatedly in separate methods before, which further ensures that cache[method](parent) is going to work consistently as expected.

The reasoning here is that writing resolvers with parent as the first argument to further cache methods is so intuitive that we even did it ourselves in the docs, without thinking about it, although its support wasn't guaranteed as #1185 demonstrates. As an example:

cacheExchange({
  resolvers: {
    Todo: {
      createdAt(parent, args, cache) {
        const date = cache.resolve(parent, 'createdAt');
        return new Date(date);
      }
    }
  },
})

Set of changes

• Add parent field to Context and ResolveInfo
• Add global ref for the current Context to be exposed
• Add store.keyOfEntity alias if entity === ctx.parent to ctx.parentKey
• Refactor store methods to decrease repetition
• Update API docs