Moving some things around

- Renamed Lambda2 -> LambdaM
- Lambda will become the Lambda that doesn't use modules
  (modules seem to limit extension/composition a lot)
- Adding Readme.md to Poly/ Lambda/ LambdaM
- Examples do not currently compile!
Working on Lambda & Monad2

- ExprD and ExprDI and ExprCore in Lambda
- A few interface changes to Lambda/TypesI2
- Added MirrorCore.SymEnv which has a simple reference
  injection using a positive map.
- Implemented MonadSym with cases for return and bind.
- A few small test cases (without sanity checks)
- Fixes to MonadTypes.