Using System.Linq.Expressions .net 4.0+ functionality

Support for the following Quotation Patterns:
- VarSet
- ForIntegerRangeLoop

Modifications to the following Quotation Patterns
- Sequence (now uses Block)
- WhileLoop (now uses Loop)

NB. No other forks, and code all seems to date back to PowerPack so I've just been doing a few things in this pull request, rather than making a series of Pull Requests. Let me know if you would prefer things done differently.