Support Enum Key Inference on Record

[sinclairzx81]

This PR adds an additional inference path for Record Keys of type Enum

enum E { A, B, C }

const T = Type.Record(Type.Enum(E), Type.Number())

type T = Static<typeof T> // type T = {
                          //   0: number;
                          //   1: number;
                          //   2: number;
                          // }

console.log(T)            // {
                          //   type: 'object',
                          //   properties: {
                          //     '0': { type: 'number', [Symbol(TypeBox.Kind)]: 'Number' },
                          //     '1': { type: 'number', [Symbol(TypeBox.Kind)]: 'Number' },
                          //     '2': { type: 'number', [Symbol(TypeBox.Kind)]: 'Number' }
                          //   },
                          //   required: [ '0', '1', '2' ],
                          //   [Symbol(TypeBox.Hint)]: 'Record',
                          //   [Symbol(TypeBox.Kind)]: 'Object'
                          // }

Fixes #646