New JSON method

[nmn]

There already seems to be a toJSON method on the prototypes of the immutable types.

There should be a new method of JSON as well.

JSON.parseImmutable

This would return an immutable values instead of a mutable one.

In case the toImmutable method is added to the prototypes, this would as simple as this, behind the scenes:

JSON.parseImmutable = (string) => JSON.parse(string).toImmutable()

The actual implementation will be smarter in that it will not throw an error if the JSON value is already immutable (string/number/bool).