Validator Alias With String Literals

[zeluisping]

Is your feature request related to a problem? Please describe.
Having to write itz.AsOptionalNumber all the time is really cumbersome, there needs to be a simpler and cleaner way.

Describe the solution you'd like
The solution would be to introduce string literals back and be able to use 'as number?' as if it was a validator. Internally the same validators would still be used.

Describe alternatives you've considered
Really I don't think there's any simpler than that.

Additional context
The new possible validation values would be:

• Primitives: 'string', 'number', 'object', 'undefined', 'null', and 'boolean'
• Primitive Arrays: 'string[]', 'number[]', 'object[]', 'undefined[]', 'null[]', and 'boolean[]'

Before

const Headers = itz.A({
    UserDomain: itz.String,
    WholeFamily: itz.AsOptionalBoolean,
    SomethingElse: itz.Either(itz.String, itz.AsNumber, itz.Undefined),
});

After

const Headers = itz.A({
    UserDomain: 'string',
    WholeFamily: 'as boolean?',
    SomethingElse: itz.Either('string', 'as number', 'undefined'),
});