Revamp Enum type generation.

[Eyas]

Inspired by #40 and, to an extent, by #37:

Given an object of name 'Foo' which has enum types, we now allow the
name 'Foo' itself to both refer to the total type (as currently), and
the prefix for the specific enum values.

So all of let x: Foo;, let x = Foo.SOME_VALUE, as well as
let.x: Foo = Foo.SOME_VALUE are valid.

Deprecate FooEnum, introducing instead temporary aliases:
export const FooEnum = Foo; and
export type FooEnum = Foo;.

We'll remove these before 1.0.

[Eyas]

See, for example, the Audience type (which is both an Enum and has member properties):

Generated TypeScript:

type AudienceBase = IntangibleBase & {
    /** The target group associated with a given audience (e.g. veterans, car owners, musicians, etc.). */
    "audienceType"?: Text | Text[];
    /** The geographic area associated with the audience. */
    "geographicArea"?: AdministrativeArea | AdministrativeArea[];
};
/** Intended audience for an item, i.e. the group for whom the item was created. */
export type Audience = "http://schema.org/Researcher" | (({
    "@type": "Audience";
} & AudienceBase) | (BusinessAudience | EducationalAudience | MedicalAudience | PeopleAudience));
export const Audience = {
    /** Researchers. */
    Researcher: ("http://schema.org/Researcher" as const)
};
/** @deprecated Use Audience as a variable instead. */
export const AudienceEnum = Audience;
/** @deprecated Use Audience as a type instead. */
export type AudienceEnum = Audience;

Resulting package .d.ts:

declare type AudienceBase = IntangibleBase & {
    /** The target group associated with a given audience (e.g. veterans, car owners, musicians, etc.). */
    "audienceType"?: Text | Text[];
    /** The geographic area associated with the audience. */
    "geographicArea"?: AdministrativeArea | AdministrativeArea[];
};
/** Intended audience for an item, i.e. the group for whom the item was created. */
export declare type Audience = "http://schema.org/Researcher" | (({
    "@type": "Audience";
} & AudienceBase) | (BusinessAudience | EducationalAudience | MedicalAudience | PeopleAudience));
export declare const Audience: {
    /** Researchers. */
    Researcher: "http://schema.org/Researcher";
};
/** @deprecated Use Audience as a variable instead. */
export declare const AudienceEnum: {
    /** Researchers. */
    Researcher: "http://schema.org/Researcher";
};
/** @deprecated Use Audience as a type instead. */
export declare type AudienceEnum = Audience;

Resulting package .js:

exports.Audience = {
    /** Researchers. */
    Researcher: "http://schema.org/Researcher"
};
/** @deprecated Use Audience as a variable instead. */
exports.AudienceEnum = exports.Audience;