Naming JSON.parsePrimitive instead of parseImmutable

[hax]

I think "immutable" is more broad, it could also mean immutable objects, people may think JSON.parseImmutable returns deep frozen immutable array/objects. So parsePrimitive may be the more accurate name.

Another reason that I don't like "immutable" in the API name, is it make us impossible to explore the possibility of "mutable" value type (like Swift, Kotlin) in the future.

[rricard]

That sounds like we'll be able to discuss this in the detached proposal for parseImmutable that we intend to detach in #330.