Implement sCU for <FastField /> (#873)

examples/FastField.js

@@ -0,0 +1,54 @@
+import React from 'react';
+import { Formik, Field, FastField, Form } from 'formik';
+
+class Input extends React.Component {
+  renders = 0;
+  render() {
+    const { field, form, ...rest } = this.props;
+    return (
+      <div>
+        {this.renders++}
+        <input {...field} {...rest} />
+      </div>
+    );
+  }
+}
+
+const Basic = () => (
+  <div>
+    <h1>Sign Up</h1>
+    <Formik
+      initialValues={{
+        firstName: '',
+        lastName: '',
+        email: '',
+      }}
+      onSubmit={values => {
+        setTimeout(() => {
+          alert(JSON.stringify(values, null, 2));
+        }, 500);
+      }}
+      render={() => (
+        <Form>
+          <label htmlFor="firstName">First Name</label>
+          <FastField name="firstName" placeholder="Jane" component={Input} />
+
+          <label htmlFor="lastName">Last Name</label>
+          <FastField name="lastName" placeholder="Doe" component={Input} />
+
+          <label htmlFor="email">Email</label>
+          <FastField
+            name="email"
+            placeholder="jane@acme.com"
+            type="email"
+            component={Input}
+          />
+
+          <button type="submit">Submit</button>
+        </Form>
+      )}
+    />
+  </div>
+);
+
+export default Basic;