Avoid unnecessary re-renders by comparing errors to previous errors

[mvestergaard]

This addresses part of #1974

Each time any field is touched (onChange, onBlur) the errors on the context changed, causing all fields to re-render once more than needed (2 renders per change)

This fix will compare the new errors object being assigned to the existing one, and only change the state if they are different.

FastField is still fundamentally broken, because it will re-render on each change to the context. Should be fixed in #2070

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[mvestergaard]

I tried my best to write a test that reproduces the problem, but it's really difficult, if not impossible in a "fake context".
Use the FastField story in the formik storybook to compare the behavior to before.