freeze functions

[mlenzen]

Write top level freeze and deep_freeze functions that take a collection and return an immutable version. deep_freeze would recursively freeze collections in the passed collection.

We could extend this idea to alter any of the three attributes mutable, ordered and unique of a passed collection. This would be similar to the collection factory except that instead of having defaults for each attribute they would be inferred from the passed collection if they weren't passed as arguments.

[mlenzen]

Should this replace the collection factory instead of being in addition to it? If a Collection is passed as the Iterable argument, inspect if for the new collections default attributes.