Introduce IOrigin to make the Dafny AST easier to work with

[keyboardDrummer]

Description

• Introduce IOrigin, which always has a Token StartToken, Token EndToken and Token Center
• Code that operates on lexical tokens now uses the Token type, instead of IToken
• Previously, an INode had .Tok and .RangeToken properties. Now it has an IOrigin that should replace both of those. Usages of .RangeToken have already been replaced, but usages of .Tok will be tackled in a follow-up PR.
• "Token Wrapping", which is used for refinement and changing error messages, which was previously done on IToken, is now done on IOrigin. Dafny.IToken no longer exists. Boogie.IToken is still there
• A RangeToken no longer takes two ITokens, which could be wrapped, but only takes two Token instances.

Future

• Replace usages of Node.tok/Tok with Node.Origin
• Rename RangeToken to SourceOrigin, and include a field for containing the "Center". Rename RangeNode to OriginNode
• Replace usages of TokenNode with RangeNode, after which RangeNode can be merged into Node
• Remove token like fields from IOrigin, so users must always choose to use Start/End/Center.
• Get rid of BoogieRangeToken

How has this been tested?

Pure refactoring, covered by existing tests

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.