Framework: Replace is-equal-shallow with shallowequal

[aduth]

This pull request seeks to resolve a performance regression caused by a bailout condition in our chosen shallow-equality library is-equal-shallow:

    if (!isPrimitive(b[key]) || !a.hasOwnProperty(key) || (a[key] !== b[key])) {
      return false;
    }

https://github.com/jonschlinkert/is-equal-shallow/blob/592a07eb802e8043493a983eee901e2b56b4b05c/index.js#L19-L20

Specifically, our data withSelect higher-order component relies on reference equality check to prevent unnecessary re-renders.

gutenberg/data/index.js

Lines 214 to 218 in 409ac98

	if ( ! isEqualShallow( nextMergeProps, mergeProps ) ) {
	this.setState( {
	mergeProps: nextMergeProps,
	} );
	}

Since the above logic will abort (and flag as unequal) if it encounters a non-primitive type, this optimization is very frequently bypassed.

The changes here swap to an alternate implementation which does not include such a bailout.

See implementation: https://github.com/dashed/shallowequal/blob/master/index.js

Testing instructions:

With React DevTools, verify that updating a block should only impact the block itself, not re-rendering large amounts of the application.

[mcsf]

🚢 🚢 🚢 🚢 🚢

[jorgefilipecosta]

This change reduces a lot the number of rerenders 👍 . The insert button still rerenders on every change e.g: every paragraph char change. But this is something to investigate and outside of the scope of this PR.