Add Symbol() support

[kanongil]

In extension of #938, I'm looking into adding support for the symbol type to Joi to allow something like this:

const enums = {
    first: Symbol('first'),
    second: Symbol('second')
};
const schema = Joi.symbol().map(enums);

schema.validate('first'); // value = Symbol(first)

This will enable eg. an options schema to declare enums using symbols instead of strings.

I could probably create it as an extension, but given that it is a native type (typeof Symbol() === 'symbol'), I'd say that it needs to be a part of the standard package.

How does this sound?

[Marsup]

It could be part of joi indeed, I'm just puzzled by map. It's a coercion from string to symbol, I think it should use Symbol.for() for that.

[kanongil]

The enums define a mapping from a string to a Symbol. In this case I have named them similarly, but it doesn't have to be that way.

You could probably also add an option for an implicit map that uses Symbol.for(), along with a pattern.

The primary use-case from my POV, is to parse a configuration. Eg. instead of having to do a string comparison to figure the option, I can test against a Symbol.

const schema = Joi.string().valid('option1', 'option2');
const config = Joi.attempt(inputConfig, schema);

if (config === 'option1') {
    …
}

vs

const option1 = Symbol();
const option2 = Symbol();

const schema = Joi.symbol().map({ option1, option2 });
const config = Joi.attempt(inputConfig, schema);

if (config === option1) {
    …
}

[Marsup]

Care to make a PR ? Do you think your map should have a case sensitive option ?