A unified method dispatch protocol for ExtensionArrays
What I'd like to have is:
1. a single, unified interface for dispatching Series methods
2. which supports EA
3. for all methods
4. and allows every method to be overridden by EA, as-needed
5. supports NA
6. doesn't require EA authors to implement some methods to signature-compatible with numpy (`round`), and signature compatible with a competing pandas version in other (`sum` via `_reduce`)
7. Clearly namespaces pandas functions, which also indicates cooperation/opt-in (perhaps via duck typing), so that 
7a. its clear which methods implements pandas interface
7b. there are no methods which are named like a numpy method, but with a pandas signature, or vice versa
7c. when reading an EA's code, you don't have to guess or figure out which code is 
a pandas method and which is an internal method for the EA (like the poorly named `_reduce`)
8. provides good error messages for unimplemented methods.