Add an entry for upcast into Ast
Manifest file for the fsharp-refactor plugin

Includes a single extension which adds the Rename refactoring
Partial implementation of Rename refactoring's binding

Currently always valid for F# files and always renames to "testName".
Implement Rename.IsValid

This function takes the arguments for the Rename refactoring as options,
so that the validity of certain arguments can be checked (i.e. checking
if the given position contains an identifier, regardless of the desired
name).
Use Rename.IsValid in MonoDevelop add-in
Use lazy identifiers to avoid unneeded computation

Particularly those involving scopeTrees or parse trees which can be
costly.
Only check position and name if both are present
Add error messages to IsValid
Move checking code into a separate module
Implement some of IsValid for AddArgument
ValidityChecking.fs forgotten two commits ago