Support an option to create immutable set

[rominf]

I need to pass ordered set as function default argument, therefore, I need an immutable set. Please provide a way to create an immutable set.

[rspeer]

I'm worried about scope creep.

An immutable version of OrderedSet would clearly be useful in a few cases, but maybe it should just be a different package. There's a lot of methods of OrderedSet that it wouldn't need.

[rominf]

But that means that lot's of code would be duplicated. Isn't possible to create a base class to implement all common functionality and create two subclasses?

[rspeer]

I'm really not interested in writing extra code for this use case. When you need a small immutable sequence that you can use as a default argument, a tuple should do just fine.

[Eric-Arellano]

Hello, rather than this being a flag on the original OrderedSet, I'd love for there to be a new FrozenOrderedSet (like frozenset but ordered).

I am happy to put up a PR for this, including comprehensive tests and type hints, given your blessing. (It's okay if you don't want to commit to it at this point - if you'd like to see the PR first before accepting the overall idea, let me know and I'd be happy to do so.)

Why do I care about this? I'm one of the maintainers of Pex and Pants. For the build tool Pants, our caching model depends on stable iteration and immutability. OrderedSet provides is this first property, and we're hoping to add FrozenOrderedSet to have both properties. We thought about inlining it, but believe FrozenOrderedSet would be helpful to others.

(Possible further context: Python is considering PEP 603 to add a builtin frozenmap: https://www.python.org/dev/peps/pep-0603/)