Variables in enums should be final.
Revert "Make this constant final and publicly accessible."

This reverts commit 66d409654843e5290c0a4071ac39d908fada5720.

There are of course advantages to accessing things via a method.
- Can implement an interface
- Can return different values based on some state

Even if this enum doesn't do it, many others do, and we may as well be consistent.
Make this final too.
_wrapperStreamers are unnecessary.
Use ImmutableMaps.
Use ImmutableMap.
This is unused.
Use samskivert's RunAnywhere to determine if we're on OS X.
More modernization.