Add meta prop to Field, FastField

Feature

Current Behavior

<Field> component, when using with a custom component or render prop, passes down the following structure to the component being rendered: { field: { name, onBlur, onChange, value }}. Extraction of touched and error for a particular field has to be done manually. This becomes quite painful when dealing with deeply nested fields. For instance, when accessing errors.foo.bar, one needs first to check the existence of errors.foo: errors.foo && errors.foo.bar.

Desired Behavior

Pass down the following structure: { field: { error, name, onBlur, onChange, touched, value }}.

Info

There is at least one caveat: <input> and other HTML elements should not receive those extra properties. It might make sense to only implement this for render prop.