Upgrade to react-redux-7, fix store instance missing issue #59

[Firenze11]

Changes

In v6 react-redux migrated from legacy react context to createContext. And in order for connect to access the store object passed from Provider, connect and Provider must come from a single react-redux dependency. Previously manifold depends on react-redux, and the embedded app also depends on react-redux, which results in connect and Provider coming from different places and caused the error in #59

1. Upgrade react-redux dependency to 7
2. react-redux in example and demo app both resolve to root level dependency
3. Moved dependency for react-redux in manifold package to peerDependency

Test Plan

Both example and demo app can render; other app that are using Manifold won't crash (after the change is published)

Checklist

• I have made this PR atomic.
• I have provided enough context for others to review.
• [ ] I have added tests to cover my changes (for features and bug fixes).
• [ ] I have updated the documentation and changelogs accordingly.
• All new and existing tests passed.

[Firenze11]

There's a similar issue in kepler keplergl/kepler.gl#351 which needs to be fixed separately.

[javidhsueh]

LGTM