Branded types now compatible with declaration option of TS

[yuhr]

Resolves #195. With this fix, the resulting static type of String.withBrand("BrandedString") will look like string & RuntypeBrand<"BrandedString">. This also makes tooltips more readable, especially for complex types, for example:

export const ID = String.withBrand('ID');
export type ID = Static<typeof ID>;

export const ArrayNonEmpty = <T extends Runtype>(element: T) =>
  Array(element).withConstraint(a => 0 < a.length || 'array must not be empty');

export const IDRequiedAndOptional = Record({ required: ArrayNonEmpty(ID) })
  .And(Partial({ optional: ArrayNonEmpty(ID) }))
  .withBrand('IDRequiedAndOptional');
export type IDRequiedAndOptional = Static<typeof IDRequiedAndOptional>;

Without this fix, the static type of IDRequiedAndOptional was being shown as:

type IDRequiedAndOptional = {
    required: (string & {
        [RuntypeName]: "ID";
    })[];
} & {
    optional?: (string & {
        [RuntypeName]: "ID";
    })[] | undefined;
} & {
    [RuntypeName]: "IDRequiedAndOptional";
}

But now:

type IDRequiedAndOptional = {
    required: (string & RuntypeBrand<"ID">)[];
} & {
    optional?: (string & RuntypeBrand<"ID">)[] | undefined;
} & RuntypeBrand<"IDRequiedAndOptional">

This is not a breaking change, because it's just an interface like following:

export interface RuntypeBrand<B extends string> {
  [RuntypeName]: B;
}